You have a contract with salary figures in it and a website offering to process it for free. Should you? The honest answer is: it depends on whether the site processes the file on their server or in your browser — and you can check which it is in about thirty seconds.
Server-side. Your file is uploaded, processed on their infrastructure, stored temporarily (per their retention policy) and then offered back as a download. This is how most well-known PDF sites work. It is usually fine for a takeaway menu and usually not fine for a signed employment contract.
Client-side. The page downloads code that runs in your browser. Your file is opened from disk into memory, modified, and written back to your downloads folder. No copy is transmitted anywhere.
If you see a POST request with a body roughly the size of your file, it was uploaded. If the only requests are for scripts and images, the work happened locally. A simpler signal: a genuinely local tool keeps working if you disconnect the network after the page has loaded.
Look for concrete statements: how long files are retained, whether they are encrypted, whether they are shared with processors, and how deletion works. Vague language like "we may use your data to improve our services" is a reason to hesitate with sensitive documents.
Server-side tools can do things browsers cannot, such as real OCR on scanned documents or converting to Word with layout preserved. Those jobs genuinely need computing power. The question is whether the job needs it — merging two PDFs does not, and that is exactly the kind of task worth keeping local.
Try it now: Merge PDF · Compress PDF