The upload portal says "maximum file size: 10 MB."

Your PDF is 48 MB. It's four pages.

So you Google "compress PDF online," click the first ad, and upload your rental contract — with your name, your signature, your salary — to a website you've never heard of, run by a company you can't name, in a country you couldn't guess.

Stop. That's the worst possible file to hand to a stranger.

Here's the thing nobody tells you: shrinking a PDF takes about thirty seconds with tools already sitting on your machine. And once you understand why PDFs balloon, you can cut file size by 80-90% without turning your document into a smudge.

Why your four-page PDF is 48 MB

A PDF is a container. Text, fonts, images, vector graphics — all packed into one file. And the weight is almost never distributed evenly.

Text is nearly free. Real PDF text is stored as characters plus drawing instructions. A hundred pages of pure text weighs less than a single photo. That's also why text stays razor-sharp at any zoom level — it's vector, not pixels.

Images are the whale. One photo dropped in from a modern phone camera can weigh more than the rest of the document combined. Print-resolution images embedded in a file that will only ever be read on a screen — that's where your 48 MB lives.

Scans are the worst case. A scanned "text" document isn't text at all. Every page is a full-resolution photograph of paper. Scan at high DPI in color and each page can weigh several megabytes — for content that's black ink on white paper.

Fonts add a slice. PDFs embed fonts so the file looks identical everywhere. Usually a minor cost, occasionally a bloated one when full font families get embedded instead of subsets.

So "compressing a PDF" almost always means one real thing: downsampling and recompressing the images inside it. Everything else is a rounding error.

The tradeoff, stated honestly

Compression isn't magic. It's a trade.

  • Downsampling throws away pixels. A 300 DPI image resampled to 150 DPI keeps a quarter of the pixel data. On a screen, you'll rarely notice. Printed and held up to your nose, you will.
  • Recompression re-encodes images with lossier settings. Photos survive this well. Fine diagrams, screenshots with small text, and anything already heavily compressed survive it less well.
  • Text and vector content are untouched. This is the part people miss. Compress a text-based PDF and the words stay perfectly crisp. Only the pictures take the hit.

Microsoft's own guidance on image compression says it plainly: compressing a picture "may make the image look different because of loss of detail" — see Microsoft's documentation on reducing picture file size. That's the deal you're making. Make it deliberately, matched to where the PDF will actually be viewed, and it's a great deal.

The rule: compress for the destination. Screen reading tolerates aggressive compression. Printing doesn't. Archiving originals tolerates none — keep an untouched copy of anything that matters before you shrink it.

First: never upload sensitive PDFs to "free" compressor sites

Think about which PDFs are ever too big. Contracts. Scanned IDs. Bank statements. Tax returns. Medical records. Signed offers.

The exact files that should never touch a random server are the exact files people upload to compressor websites every day.

You have no idea how long that site keeps your file, who can read it, or what jurisdiction it sits in. Their privacy policy says files are deleted "after processing." You're trusting a free website, with your signed contract, on its word.

I've written before about why uploading files to random websites is a habit worth breaking, and why offline conversion beats online converters every single time. PDFs are the sharpest version of that argument. Everything below runs on your own machine. Nothing leaves it.

Mac: the 30-second method (Preview)

Preview has done this for years and nobody notices.

  1. Open the PDF in Preview.
  2. Choose File > Export (not Export as PDF).
  3. In the Quartz Filter pop-up menu, select Reduce File Size.
  4. Save — either replace the original or give it a new name.

That's it. Apple documents the steps in its Preview user guide, along with the honest caveat: the compressed PDF may be lower quality than the original.

Fair warning from real use: Preview's built-in filter is aggressive. On image-heavy files it produces dramatic size cuts, and on files with photos you care about, it can visibly soften them. For screen-only reading and portal uploads, it's usually fine. For anything going to print, use the Ghostscript method below, where you control the dial.

Windows: the built-in route

Windows has no one-click equivalent of Preview's filter, but you have two solid offline options.

Microsoft Print to PDF. Open the PDF in any viewer, print it, and choose Microsoft Print to PDF as the printer. This re-renders the document, which often shrinks bloated files. Crude but effective, and it's already installed.

Fix it at the source in Word. If the PDF comes out of a Word document, compress the images before export: select a picture, then Picture Format > Compress Pictures, and pick a lower resolution. Microsoft notes the default resolution in Microsoft 365 is 220 ppi, and you can go lower for email and web use. Then export to PDF. A document built from compressed images produces a small PDF without any post-processing.

The power tool: Ghostscript (Mac, Windows, Linux)

This is the method the others imitate. Ghostscript is free, open source, runs everywhere, and gives you an actual quality dial instead of a single mystery button.

One command:

gs -sDEVICE=pdfwrite -dPDFSETTINGS=/ebook -dNOPAUSE -dQUIET -dBATCH -sOutputFile=small.pdf input.pdf

The -dPDFSETTINGS presets are the dial. Per the Ghostscript documentation:

Preset Color/gray images Meant for
/screen 72 DPI Smallest files, screen-only reading
/ebook 150 DPI The sweet spot — small, still clean on screen
/printer 300 DPI Print-quality output
/prepress 300 DPI Professional printing, color preserved

/ebook is the default answer. It downsamples color and grayscale images to 150 DPI — plenty for laptops, phones, and portals — and routinely collapses scan-heavy monsters to a fraction of their size while leaving text vector-sharp.

If /ebook output looks too soft for your use, rerun with /printer. Ten seconds per attempt, original untouched. That iteration loop is exactly what one-button tools can't give you.

On a Mac, install it with brew install ghostscript. On Windows, grab the installer from the official site.

iPhone: shrink the inputs, not the output

iOS doesn't ship a visible "compress PDF" button, so the smart play on a phone is to build smaller PDFs in the first place — the biggest lever is the images going in.

Most phone PDFs start life as photos: receipts, documents, whiteboards, forms. Full-resolution camera images stuffed straight into a PDF is how a two-page file hits 20 MB.

This is where ConvertApps earns its spot on my home screen. It converts images and builds PDFs entirely on-device — convert HEICs to compact JPGs, batch a stack of them, turn them into a PDF, done. Nothing uploads anywhere, which is the whole point when the pages are your lease or your kid's medical form. The privacy angle isn't a feature bullet; it's the reason the app exists.

And if the document is paper, scan it properly instead of photographing it — I covered why in your phone camera is a scanner. A scan cropped to the page, rendered as a document instead of a 12-megapixel photo, is smaller and more legible at the same time.

Pick the DPI for the destination, not the vibe

Every method above is secretly the same decision: how many dots per inch do the images inside your PDF actually need?

Here's the honest ladder, anchored to the same numbers Ghostscript's presets use:

  • 72 DPI — thumbnail territory. Fine for a quick reference nobody will zoom into. This is why /screen output sometimes looks rough: it's built for exactly that, the smallest possible file.
  • 150 DPI — the workhorse. Clean on laptops, phones, and tablets at normal zoom. This is /ebook, and it's the right answer for portal uploads, email attachments, and anything read on a screen.
  • 300 DPI — print quality. If someone will hold paper, stay here. File sizes drop less, but nothing visibly degrades on the page.

Notice what's missing: anything above 300. Phone cameras capture images far denser than any of these targets, which is exactly why untreated photo-PDFs are enormous. You're paying storage for resolution no reader will ever see.

One more trap worth naming: compressing twice. Lossy compression stacks. Run a PDF through one aggressive tool, then another, and the artifacts multiply — muddy photos, halos around text in scans. Compress once, from the original, at the level the destination needs. If the result is too soft, go back to the original and compress lighter. Never re-compress the compressed copy.

And if you receive a bloated PDF from someone else? Same rules. Save their original untouched, make your smaller working copy, and if it's headed anywhere sensitive, keep the whole operation offline.

The decision table

Situation Do this
Mac, need it small fast Preview > Export > Quartz Filter > Reduce File Size
Mac/Windows/Linux, want control Ghostscript with /ebook, escalate to /printer if soft
PDF made from Word Compress Pictures in Word first, then export
Windows, no installs allowed Print to Microsoft Print to PDF
On iPhone Convert/shrink images on-device, then build the PDF
Scanned document, still huge Rescan in grayscale or black-and-white at lower DPI
Contract, ID, or anything signed Any of the above — never an online compressor

Two habits that quietly prevent the problem:

  1. Scan text documents in black and white or grayscale. Color scans of black-text pages are pure waste. This alone routinely cuts scan size massively before any compression tool touches the file.
  2. Keep the original. Compression is one-way. You can always make a smaller copy from the master; you can never recover detail from the compressed one.

FAQ

Does reducing PDF file size lower the quality?

For images inside the PDF, yes — that's literally what compression does. Downsampling and recompression trade image detail for size. Text and vector graphics are not degraded; they stay sharp at any zoom. Whether you notice depends on the destination: 150 DPI images look fine on screens and rough in print.

How do I reduce PDF size without losing quality at all?

Truly lossless wins are limited but real: rebuild the file so fonts are subset and unused objects are stripped, or fix bloat at the source by compressing images before the PDF is created. If the file is a scan, rescanning in grayscale at a sane DPI beats any after-the-fact tool. Beyond that, meaningful size cuts require lossy image compression — there's no free lunch.

Why is my scanned PDF so huge?

Because it isn't text — it's a photo album. Every scanned page is stored as a full image, and color scans at high DPI cost megabytes per page. Rescan in black and white or grayscale at lower DPI, or run the file through Ghostscript's /ebook preset, which downsamples those page images to 150 DPI.

Are online PDF compressors safe?

Treat them as if they are not. You're transmitting the full contents of your document to a third-party server with no way to verify deletion, access, or jurisdiction. For a meme, who cares. For the contracts, statements, and IDs that are usually the oversized files in question, use an offline tool. Every method in this guide runs locally — and on your phone, ConvertApps covers the image and PDF side without a single byte leaving the device.

What's the best target size for emailing a PDF?

Aim for single-digit megabytes. Mail providers reject large attachments outright, and even when they go through, they choke inboxes. Ghostscript's /ebook preset or Preview's Reduce File Size filter typically gets a normal document well under that bar. If a file still won't shrink enough, it's image-heavy by nature — share a link instead.

The takeaway

A big PDF is an image problem wearing a document costume — shrink the pictures with a tool on your own machine, and never hand your contracts to a free website to do it.

— Dolce