Why is my PDF so large, and how do I shrink it?
How to tell which kind of PDF you have
Try to select a word with your finger or cursor. If a text selection appears, the page is real text and it is already small. If you can only draw a box around the whole page, it is an image — and that is where your megabytes are.
This one test decides everything that follows. Text PDFs barely respond to compression; scanned PDFs can often drop by 70–90% with no visible difference at reading size.
What quality setting to choose
For documents meant to be read rather than printed, medium quality is almost always enough. Text stays crisp because the compression artefacts appear in smooth areas, not on high-contrast edges. Go to the lowest setting only for throwaway copies — receipts you are emailing once.
If the document will be printed or archived, keep the original and send the compressed copy. Compression cannot be undone.
Things that do not shrink a PDF
Renaming, zipping (PDFs are already compressed internally, so a zip saves almost nothing), and deleting bookmarks. People try all three. The size is in the pixels.
One real saving that is often missed: if a scan is in colour but the original is black text on white paper, converting to greyscale before compressing removes two thirds of the data before any quality loss is involved.
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