What actually makes a PDF huge, which compression settings matter, and how to hit an email size limit without turning your document into mush.
Almost every PDF that is too large is too large for one of three reasons, and only one of them is really about compression. Understanding which reason applies to your file is the difference between a clean 80% size reduction and a blurry document that is still 12 MB.
Why your PDF is big in the first place
- Embedded raster images. A phone photo dropped into a report is often 4–8 MB on its own, at a resolution far beyond what any printer or screen needs.
- Scanned pages. A scanner produces one full-page image per page. A 40-page scan at 600 DPI is a 200 MB file before you have written a word.
- Embedded fonts and duplicated resources. Fonts embedded in full rather than subset, or the same logo re-embedded on every page instead of referenced once.
Text itself is almost never the problem. A hundred pages of pure text is typically under 1 MB. If your text-only document is 30 MB, something else is riding along inside it.
The three levers compression actually pulls
Image downsampling
This reduces the pixel dimensions of embedded images to a target DPI. For screen reading, 150 DPI is generous. For office printing, 200–300 DPI is plenty. Anything above 300 DPI is invisible to the human eye at normal reading distance and is pure file weight.
Image re-encoding
Photographs compress well as JPEG; line art, screenshots and text-heavy scans do not — they get ugly halos. If your document is mostly screenshots or diagrams, an aggressive JPEG setting is exactly the wrong choice, and you will see it immediately in the edges of text.
Object stream cleanup
Removing unused objects, deduplicating repeated resources, subsetting fonts and compressing the cross-reference table. This is lossless. It typically wins 5–20% and never costs you quality, so there is no reason not to do it.
A practical recipe by goal
- Email attachment under 10 MB: downsample to 150 DPI, medium image quality, lossless cleanup on.
- Web download or portfolio: 120–150 DPI, medium quality. Prioritise fast loading over print fidelity.
- Client deliverable that will be printed: 300 DPI, high quality. Accept a bigger file; a pixellated logo on a printed proposal costs more than bandwidth.
- Archival or legal filing: do not aggressively compress at all. Use lossless cleanup only, and consider PDF/A instead.
The mistake almost everyone makes
Compressing the same file repeatedly. Lossy image compression is generational: each pass throws away information the previous pass already degraded. Two rounds of medium compression look far worse than one round of aggressive compression at the same final size. Always compress from the original, not from a compressed copy.
When compression is not the answer
If your file is a 300-page scan, the real fix is OCR plus a rebuild, not compression — or simply splitting the document so recipients only get the part they need. If your file is large because it contains a full-resolution photographic appendix, extract the appendix into a separate download and link to it. Sending someone a 90 MB file so they can read a two-page cover memo is a workflow problem, not a compression problem.
How to check your work
Open the compressed file at 200% zoom and look at three things: the smallest body text, any thin table rules, and any logo with a gradient. Those three fail first. If they hold up at 200%, they will hold up in print and on screen.
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