Why PDF work belongs in the browser
A PDF is usually the most sensitive file on the desktop. It is the signed lease, the payslip, the scan of a passport, the diagnosis letter. The standard advice — drag it into whichever free converter ranks first — asks you to send exactly that document to an unknown server, where retention policies are a paragraph in a footer rather than something you can verify.
None of that is necessary. pdf.js can parse and render any PDF in the browser, and pdf-lib can write one back out, which covers merging, splitting, rotating, page numbering, watermarking, form filling and signing entirely on your own machine. The tools here are built on those libraries, running in Web Workers, so a two-hundred-page document is handled locally and never leaves the tab.
Compression, and the trade-off it makes
PDF compressors fall into two families. One rewrites the file structure and downsamples embedded images while keeping the text layer intact; the other rasterizes each page into an image and rebuilds the document around it. The tool here does the second, because the engines that do the first well are licensed in ways that would force this site to change how it is distributed.
That trade-off is worth stating plainly: compressed output is image-based, so the text is no longer selectable or searchable. For an upload portal that only cares about kilobytes — the common case — that is fine, and the size reduction is substantial. When you need the text layer preserved, keep the original and submit that instead, or run the compressed file back through the OCR tool to add a searchable layer.
Passwords, signatures and legal weight
Two different things get called "unlocking" a PDF. Removing a known open password — the one you type to view a document your bank sent you — is a decryption you are entitled to perform, and the unlock tool does it locally so the password itself is never transmitted. Removing owner restrictions on a document you did not create is a different question, and the tools here are not built to defeat encryption you do not have the key for.
Signing works the same way: the signature you draw or type is drawn into the page as visible artwork, which is what the overwhelming majority of forms, leases and consent documents actually ask for. It is not a cryptographic digital signature backed by a certificate authority, so where a jurisdiction requires a qualified electronic signature, use a certified provider.