A PDF that's 50MB when it should be 2MB is a genuine nuisance. Email clients bounce it, upload forms reject it, sharing it is slow. The usual culprit is images — scanned documents and photo-heavy PDFs balloon in size because the images were captured at much higher resolution than you need.
Compression brings the file size down by reducing image resolution and stripping unnecessary internal data. The trick is choosing the right compression level so you don't make the document look bad in the process.
The three levels and when to use each
Light compression is for documents where visual quality matters a lot — signed contracts, architectural drawings, anything with fine print or detailed graphics. It cuts file size modestly (typically 10-30%) while keeping everything looking essentially identical.
Medium compression is what most people need. It typically cuts 40-60% of the file size, and the difference in quality is barely visible on screen or in print. This is the right choice for reports, presentations, and forms.
Strong compression makes the file as small as possible. Images get noticeably softer, especially at close zoom. Fine for archiving or sending something where the recipient just needs to read the text.
What actually gets compressed
The main thing the tool does is downsample images — reducing their resolution from, say, 300 DPI to something web-appropriate. It also removes duplicate font resources and unused objects that accumulate in PDFs over time (especially files that have been edited or converted multiple times). And it recompresses the internal data streams.
What it doesn't touch: the text content, bookmarks, form fields, or the logical structure of the document. Text in a compressed PDF stays sharp because it's stored as vector data, not as an image.
When compression won't help much
If your PDF is large because it has hundreds of pages but not many images, compression will have limited effect — maybe 5-15%. The file is big because there's a lot of content, not because anything is wasteful.
Similarly, if the PDF was already compressed by another tool, you might not see much additional reduction. There's a floor to how small a given document can get.
The one case where strong compression is worth it
Email attachments. Most email servers cap attachments at 25MB. If your PDF is over that, strong compression is usually the fastest fix. Yes, images will be slightly softer, but for a document someone's reading on screen, it genuinely doesn't matter.
What if it's still too large?
Split the PDF into sections and send them separately. Or, if the document came from a source file (Word, PowerPoint, InDesign), re-export it from there with lower image quality settings. The compression tool works on what's already in the PDF — if you can reduce resolution at the source, that's always better.
Does it look different when printed?
Light and medium compression are essentially invisible in print. Strong compression may show image softness on a high-quality printer, but for standard office printing it's usually fine. If print quality is critical, stick with light compression and verify the output before sending to print.