Compression shrinks images. It reduces their resolution and re-encodes them at lower quality. Everything else in a PDF is already small and already compressed.
So what to expect depends entirely on what is inside: a scanned document can lose most of its size, and a text-only export from Word may barely change. If a text PDF is huge, compression is the wrong tool and something else is going on.
Most advice about compressing PDFs skips the one fact that predicts the outcome. A PDF is not one kind of thing. It is a container, and what compression can do to it depends on what got put inside.
What is taking up the space
Open any large PDF and the weight is in one of two places. Either it is full of images, or it has an unusual amount of embedded font data. In practice it is nearly always images.
Text is cheap. A page of prose is some glyph codes, some positioning numbers and a reference to a font, all deflate-compressed. A full-page photograph is millions of pixels. At 300 DPI an A4 page is 2480 by 3508 pixels, and at three bytes a pixel that is about 26 MB before the scanner compresses it. One image page can outweigh a long text document, which is why page count tells you almost nothing about file size.
Glyph codes and coordinates. Already compressed inside the file.
2480 by 3508 pixels at three bytes each, before any compression. This is where the file size lives.
Recommended mode brings every image to 150 ppi at JPEG quality 0.75 and never touches the text.
What compression does
Three things, in roughly this order of impact.
Downsampling removes pixels
A 600 DPI scan holds four times the pixels of a 300 DPI one and looks no better on a screen. Reducing resolution to match how the document will be used is the largest single saving available.
Re-encoding lowers JPEG quality
Each image is decoded and written back out at a lower quality setting. This is where visible artifacts come from, and it is the step to be careful with on pages containing text.
Housekeeping removes what is unused
Duplicate images referenced once each, embedded thumbnails, orphaned objects, unused font glyphs and metadata. Small individually, occasionally significant in files that have been edited many times.
Compression modes, and which to pick
The mode names describe how aggressive the downsampling and re-encoding are. The right choice follows from where the document is going, not from how big it currently is. In the PDFslime compressor, Lossless only tidies the file structure, Recommended brings images to 150 ppi, Strong to 110 and Extreme to 72, and Scan rebuilds each page as a JPEG with a searchable text layer.
- Will be printedLossless, or a printed proof first
- Read on screen onlyStrong is usually fine
- Email attachmentRecommended, then Strong if needed
- Archival copyDo not compress at all
- Legal or medical recordLossless, and keep the original
- Web downloadStrong
Telling when you have gone too far
Compression artifacts show up in exactly the place people do not look. Zooming out makes a compressed page look fine. The damage is at actual size, on the smallest text.
JPEG compression is built for photographs, where gradual tonal change is the norm. Text is the opposite: hard black edges against white. Compressed too hard, those edges grow a faint grey halo and the letters lose their crispness. On small text this crosses from unattractive to unreadable quickly. Why a PDF looks blurry after compression shows how to tell which setting did it.
Why compressing twice makes things worse
Lossy compression discards information permanently. Running a second pass over an already compressed file does not recover any of it, and the second pass now works on the artifacts introduced by the first, treating them as real detail worth preserving.
The result is a file that is barely smaller and noticeably worse. If a compressed file is still too large, the right move is to go back to the original and compress harder in one pass, not to compress the output again.
How to get the best result
- Artifacts are introduced once, at a known level
- The compressor sees real image detail to work with
- Predictable quality across every page
- Easy to step back and retry at a gentler setting
Why it goes wrong
- Artifacts from the first pass get treated as detail
- Size barely improves, quality clearly degrades
- Different pages degrade by different amounts
- No way back if the original was overwritten
What to do when compression is not the answer
Sometimes a file cannot reach the size you need while staying readable. The document is carrying more information than the target allows, and no compressor changes that.
Three alternatives are usually better than compressing into unreadability. Split the document and send it in parts, which is routine in legal work. Re-export from the original source with better settings, since a PDF exported from Word with images at full resolution is often much larger than it needs to be. Or remove what does not need to be there, starting with blank scanned pages, which are pure weight and cost nothing to lose.
Compress a PDF in your browser
Five modes, from Lossless to Scan, with the file never leaving your machine. No upload, no queue, no sign-up.
Common questions
What people want to know before they trust the output.
01How much can compression shrink a PDF?
It depends entirely on what is inside. Scanned and image-heavy documents can lose most of their size, because there is a great deal of image data to reduce. Text-only PDFs exported from a word processor may barely change. When PDFslime's compressor finds nothing to save, it hands back the original rather than a larger copy.
02Does compression reduce quality?
Image quality, yes. Real text stays sharp because it is not an image. Two exceptions: on a scanned page the text is part of the picture and degrades with it, and PDFslime's Scan mode deliberately re-renders each page as a picture, keeping an invisible text layer so the words stay searchable.
03Which mode should I use?
Recommended first, for almost anything. If the file is still too big, Strong, then Extreme. Use Lossless for a document that will be printed or archived, since it leaves images alone, and Scan for pages that are photographs of paper. Check the smallest text at 100 percent zoom before sending.
04Why will my PDF not get any smaller?
Almost certainly because it contains no images to compress. A text-only document is already about as small as it can be. If it is nonetheless large, look for embedded font subsets, an invisible OCR text layer, or a high resolution image placed at small size on the page.
05Can I compress the same file twice to make it smaller?
You can, and you should not. The second pass works on artifacts left by the first and treats them as real detail, so the file gets meaningfully worse for very little size saving. Go back to the original and compress harder in one pass instead.
06Can I compress a password-protected PDF?
Not while it is encrypted, because the compressor cannot read the image data it needs to work on. Remove the password from a document you own, compress the unlocked copy, and re-apply protection afterwards if the file still needs it.