PDF tools
PDF work happens in this tab through two independent engines: one that edits the document structure, and one that renders pages to images. Which one runs matters a great deal to the result, so the tools say which they are using.
Popular tools
Two ways to make a PDF smaller
Structural optimisation rewrites the file: it drops document metadata and packs the objects more efficiently. It is lossless and it keeps text selectable, links clickable and forms usable — but on a document that is already tidy it may save almost nothing.
Rasterised compression renders each page to an image and rebuilds the document from those images. It shrinks scanned documents dramatically. It also destroys searchable text, links, form fields, annotations, layers, accessibility structure and any digital signature. The tool warns before it runs, not after.
Encrypted and signed documents
A password-protected PDF cannot be edited structurally, and this site will not work around that by ignoring the encryption. If you supply the password, a flattened rasterised copy can be produced instead — with the same loss of text and structure that always implies.
Editing a digitally signed PDF invalidates the signature. That is how signatures work; any tool claiming otherwise is either not really editing the file or not really preserving the signature. You are asked to acknowledge it before the edit runs.
Page limits
Rasterisation renders one page at a time and releases it before starting the next, which is what keeps a 400-page document from exhausting the tab. The page ceiling still depends on your device: a phone is capped far lower than a desktop, and the limit is applied before your file is read rather than after the tab has already run out of memory.