A 16-page 200 dpi scanned PDF went from 16.91 MB to 1.54 MB (90.9% smaller) in 1.01 seconds on our production server, using Ghostscript's /screen preset. That is the single number most people are looking for, measured rather than quoted.
Below is the full table we measured, including the preset that does *nothing* (which is the more useful finding): /ebook produced 16.91 MB - identical to the input - because the source pages are already 150-200 dpi images, so the preset's 150 dpi target had nothing to remove. Compression only helps when it actually downsamples.
Frequently asked questions
What compression ratio should I expect for a scanned PDF?
Expect roughly 5-10x on high-resolution scans (our measured case: 16.91 MB to 1.54 MB, 90.9%) and almost nothing on files that are already low-resolution. Vector/text PDFs behave differently again: they shrink little and lose no quality.
Which Ghostscript preset is best for email and web forms?
Use /screen (72 dpi images) when the file must fit a portal limit such as 100 KB-2 MB; use /ebook (150 dpi) when the document must stay readable on screen but the hard limit is looser. /printer (300 dpi) is for print and saves the least.
How was this benchmark measured?
On our own server (Linux, Ghostscript 10, Python 3.11) with a synthetic 16-page invoice-style scan at 200 dpi, timed with the shell's own timer, three runs averaged. The exact commands are listed on this page so you can reproduce it.
Does compressing a PDF reduce its page count or text?
No - recompression only rewrites the image streams. Page count, page size and any existing text layer are preserved (verified with qpdf --check and page counting after each run).
Do it in the full tool
Open Compress PDF for all options, or jump straight to another step below.