Shrink
Reduce the size of a PDF
A PDF too large for an attachment, for an online submission, for sending on. Before compressing, it is worth knowing where the size comes from: the answer decides what you can hope to save.
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Where the size of a PDF comes from
Almost always from images. A document of text and vectors (a contract, a quote, a word-processor export with no illustration) takes a few hundred kilobytes for a hundred pages. A ten-page scan is ten times larger, because every page in it is a photograph. In between, a presentation export often carries the same background repeated on every slide: the same bytes, a hundred times over.
What compression does, and what it will not do
It rewrites the document structure in compressed form, which is lossless. It resamples the embedded photographs, which is not. It reduces repeated identical images to one copy, which costs nothing. What it never does is turn text into an image. Text stays selectable, searchable and readable by a screen reader after compression; a document whose text had become a picture would have lost far more than a few megabytes.
When compression saves nothing
If the file held no heavy images, there is nothing to resample and the saving is limited to the structure. Three other options remain. Extract pages, if you only need part of the document. Crop, if wide white margins are being scanned at full resolution. And for a scan that was never OCR'd, OCR does not make it smaller but does make it searchable, which is often the real request hiding behind “it is too big”.