How PDF compression works in the browser

A PDF is a container of drawing instructions, fonts and embedded images. Making it smaller means either rewriting that container more tightly, or replacing the pages with lighter images. Those two routes behave differently.

Light rebuild

A light rebuild copies the document into a new file and keeps the text as text. You can still select and search it. The saving is often modest, because the large parts of a PDF are usually the photographs already embedded in it.

Redrawing pages

The smaller presets draw each page and save it as a JPEG. That routinely cuts the size of scanned documents and image-heavy files. The trade is that the text becomes part of the picture: it cannot be selected, and small type depends on the preset. High keeps more detail. Small favours file size.

RS Labs does this in the tab you already have open, and limits image compression to 40 pages so the browser is not asked to hold an entire book in memory. Split a longer file first.