How it works
Every other PDF site asks you to upload a document to a machine you know nothing about. This one does the work where the file already is — and that choice explains both what it does well and where it stops.
The work happens in your browser
The PDF engine is a Web Worker: a second thread inside the tab. It opens documents, moves pages, embeds images, fills forms and writes the file back out, all in memory. Page rendering uses pdf.js, the same engine Firefox uses to display PDFs, and password protection uses qpdf compiled to WebAssembly.
Because the heavy work sits on a worker, the interface keeps responding while a hundred-page scan is processed, and a job can be cancelled the moment you change your mind.
Why this way
Uploading is the part that costs you privacy and costs a service money. Removing it removes both: your document stays yours, and there is no per-file bill to fund with accounts, tiers or ads.
It is also usually faster. A 40 MB scan never travels twice across a network before you see the result.
What it costs you
Your machine does the work, so very large documents are limited by memory rather than by a plan, and a slow computer is a slow conversion. The tab has to stay open: navigating away cancels the job, because there is nothing running anywhere else to finish it.
Browser compression cannot match what a desktop program with Ghostscript achieves on text-heavy files, and a few formats — Office documents, PDF/A, OCR — cannot be done in a browser at all.
The four tools that need a local service
Word to PDF, PDF to Word, PDF/A and OCR use LibreOffice, Ghostscript and Tesseract. Those are real programs, not something a tab can host, so they run in a small service on your own computer and the browser talks to it directly.
This works when you run Dosya Kalemi on your own machine. It does not work from this public site: browsers deliberately block a public page from opening connections to services on your computer, and that rule is a good one even though it costs us four tools here.
Which browsers
A current version of Chrome, Edge, Firefox or Safari. The tools lean on WebAssembly, Web Workers and canvas, all of which have been standard for years; an old browser will fail loudly rather than produce a broken file.