PDF Compression Explained: Screen vs eBook vs Printer (With Real Test Results)
Each level is a fixed recipe for shrinking the images inside a PDF. Knowing the recipe tells you in advance which level will actually make your file smaller -- and which won't.
What actually makes a PDF big
Typed text takes up very little space in a PDF: each letter is stored as a character code plus a reference to a font, not as a picture. Lines, charts and other vector graphics are similarly compact. A 20-page report of plain text is often well under a megabyte.
Images are different. A single photo taken on a phone can be several megabytes, and a scanned document is nothing but images -- every page is one picture of paper. In practice, when a PDF is too big to email or upload, embedded images are almost always the reason. That's why every compression level works on images and leaves the text alone.
What happens when you press Compress
Our Compress PDF tool rewrites your file with Ghostscript, a long-established open-source PDF engine, using one of its three standard presets: /screen, /ebook or /printer. Each preset combines two techniques.
Downsampling lowers the resolution of images that are sharper than the preset needs. Resolution is measured in dots per inch (dpi) -- how many pixels the image uses for each inch of the printed page. Halving the resolution leaves roughly a quarter of the pixels, which is where most of the saving comes from.
Re-encoding saves photographic images with JPEG compression, which discards fine detail the eye is unlikely to notice in exchange for a much smaller file.
Text stays as real text at every level, so it remains sharp at any zoom and can still be selected and searched.
The three levels, in numbers
The figures below come from Ghostscript's own documentation for the three presets our tool uses.
Screen: colour and greyscale images are reduced to 72 dpi, and black-and-white images to 300 dpi. It also uses the stronger of the two JPEG settings. 72 dpi is fine on a screen but looks soft or blocky when printed.
eBook (our default): colour and greyscale images are reduced to 150 dpi, black-and-white images to 300 dpi, with the same JPEG setting as Screen. A 150 dpi image generally still prints acceptably at normal size, so this is the sensible balance for most documents.
Printer: the preset lists 300 dpi (1,200 dpi for black-and-white), but it has downsampling switched off, so images keep their original resolution. It also uses a gentler, higher-quality JPEG setting. The result looks almost identical to the original -- and is usually only slightly smaller, if at all.
| Level | Colour and greyscale images | Black-and-white images | Colour images reduced only if above | JPEG setting | Best for |
|---|---|---|---|---|---|
| Screen | 72 dpi | 300 dpi | 108 dpi | Stronger | Reading on screen only |
| eBook (default) | 150 dpi | 300 dpi | 225 dpi | Stronger (same as Screen) | Email and most documents |
| Printer | Not reduced (keeps original) | Not reduced | No downsampling | Gentler, higher quality | Professional printing |
Source: Ghostscript's documentation for the /screen, /ebook and /printer presets used by our Compress PDF tool.
The 1.5x rule: why some images aren't touched at all
Ghostscript only downsamples an image when its resolution is more than 1.5 times the target. With the eBook target of 150 dpi, that means only images above 225 dpi are reduced; with Screen's 72 dpi, anything above 108 dpi is reduced.
This explains a common surprise. Suppose your scanner saves pages at 200 dpi. That's below eBook's 225 dpi threshold, so eBook leaves those images as they are and the file hardly changes. Only Screen shrinks it. A 300 dpi scan, on the other hand, is reduced by both Screen and eBook.
What we measured
We ran three typical files through the same Ghostscript presets our tool uses (Ghostscript 10.02, September 2026). Your own results will depend on what's inside your PDF, but the pattern is consistent.
A PDF containing one 3000 x 2000 pixel photo at 300 dpi (3.2 MB): Screen produced 50 KB (1.6% of the original, photo now 72 dpi); eBook produced 127 KB (3.9%, photo now 150 dpi); Printer made no meaningful difference -- the photo kept its full 300 dpi.
A one-page greyscale scan at 200 dpi (93 KB): Screen produced 24 KB (25%). eBook and Printer didn't reduce it at all, because 200 dpi is below their downsampling threshold -- Ghostscript's output was actually 6-9% larger.
A five-page, text-only PDF (4.5 KB): every level produced a larger file -- about 25% larger on Screen and eBook, and 170% larger on Printer, which copied the font into the file. The text itself stayed as real, selectable text at every level.
| Test file | Original | Screen | eBook | Printer |
|---|---|---|---|---|
| One photo, 3000 x 2000 px at 300 dpi | 3.2 MB | 50 KB (1.6%) | 127 KB (3.9%) | No meaningful change |
| One-page greyscale scan at 200 dpi | 93 KB | 24 KB (25%) | Not reduced (6-9% larger) | Not reduced (6-9% larger) |
| Five-page, text-only PDF | 4.5 KB | About 25% larger | About 25% larger | About 170% larger |
When the result isn't smaller than your upload, our tool gives you back your original file instead.
Why a "compressed" file can come out bigger -- and what we do about it
Ghostscript's developers state plainly that rewriting a PDF is not guaranteed to make it smaller, and can make it larger. There's no image data to shrink in a text-only file, and rewriting the file can add overhead -- for example by copying fonts into it, as Printer did in our test.
So our tool compares the result with your upload. If the compressed version isn't smaller, you get your original file back unchanged instead of a bigger one. If that happens, your PDF is already about as small as these presets can make it.
Which level to choose
Documents with photos that you'll email or read on screen: start with eBook. Try Screen only if you still need a smaller file and nobody will print it.
Scanned documents: eBook works well on 300 dpi scans. For 200 dpi scans only Screen reduces the size -- but a scanned page is a picture of text, so at 72 dpi small print can become hard to read. Open the result and zoom in before you send it.
Anything that will be printed professionally: use Printer, and expect only a small reduction.
Text-only PDFs such as contracts or letters: these are usually small already. Compression won't help much -- you'll most likely get your original back.
A quick checklist before you send the file
Check the new file size against the limit you're trying to meet, open the compressed PDF and zoom in on the smallest text and on any photos, and keep your original file until you're happy with the compressed copy. Your uploaded file and the result are deleted from our server as soon as your download is ready, so we don't keep a copy for you.