'Your attachment exceeds the maximum size limit.' That message has caused more last-minute stress than almost anything else in office life. You've exported a perfectly normal report and somehow it's 47MB. The email limit is 25MB. The client needs it in ten minutes.
PDF compression sounds technical but it's genuinely one of the simpler file tasks once you understand what's actually making the file large. Spoiler: it's almost always images.
Why PDFs get so big
Text is cheap in terms of file size. A hundred pages of pure text might be a few hundred kilobytes. What balloons PDFs is embedded images.
When you export a Word document that has five screenshots in it, those screenshots get embedded at their original resolution. If you took them on a high-DPI display, each one might be 2-3MB. A ten-page report with a dozen screenshots can easily hit 30MB before you've done anything wrong.
Scanned documents are even worse because the whole page is an image. A document scanned at 300 DPI produces enormous files. Most people don't need 300 DPI for readable text - 150 DPI is plenty for anything that won't be printed large, and it's four times smaller.
Fonts are a smaller but real contributor. PDFs embed the full font files used in the document to make sure they look right on any system. If you've used several fonts, including any that have large character sets for non-Latin scripts, that adds up.
What compression actually does
Most PDF compression tools do one or more of these things: they reduce the resolution of embedded images (downsampling), they re-encode images at higher compression (lower quality setting), and they strip metadata and other overhead data that most readers don't need.
The downsampling is usually the biggest win. Dropping image resolution from 300 DPI to 150 DPI cuts the image data to 25% of its original size. On a PDF with a lot of screenshots or photos, this alone can reduce file size by 70-80%.
Re-encoding at lower quality adds a bit more savings at the cost of some visual quality. At reasonable settings you won't see the difference on screen. At aggressive settings you'll start to notice artifacts, especially in photos.
How much can you safely compress?
It depends entirely on what the PDF is for.
If it's a report or presentation that people will read on screen and never print, you can compress pretty aggressively. Target under 5MB and check that text is still readable - that's the main thing.
If it's going to be printed, you need to keep enough resolution for the images to look good at print size. Compressing too much here produces blurry prints, which looks unprofessional and reflects badly on whatever it is you're sending.
Legal documents, contracts, anything that might be used as official evidence - compress conservatively. Readability and integrity matter more than file size.
Which tools to use
Adobe Acrobat is the gold standard if you have access to it. It shows you exactly what's making the file large and gives you fine-grained control over every compression setting. If you're dealing with PDFs regularly, it's worth having.
For everyone else, browser-based tools work well for the common case. You upload, choose a compression level, download the result. Most handle 90% of situations without any issues.
One thing to watch for with online tools: if you're compressing something sensitive - a contract, a medical document, anything with personal information - make sure the tool processes it locally in your browser. Some tools do everything client-side with JavaScript, which means your file never leaves your device. Others upload to a server. The privacy policy should say which one it is. If it doesn't say, assume server-side.