You type "merge PDF", click the first result, pick your files and a few seconds later the result is ready. But where did your files go during those few seconds? If what you merged was a lease, a payslip, a passport copy or lab results, that's not a small question.
Most online PDF tools upload your file to their server, process it there and send the result back. This guide shows how to tell, in two minutes and with no technical knowledge, whether a tool uploads your files. Two tests and a browser; that's all.
Are online PDF tools safe?
The short answer: most well-known sites are serious companies that say they delete files after a while, and they very likely do. But for that while, your file is out of your hands: in transit, waiting on a server, perhaps in backups or error logs.
A tool that runs in the browser is different. When you open the site, only the program itself is downloaded. When you pick a file, the work happens on your own computer's or phone's processor, in the browser's memory. The result is created on your device too. The document's contents go nowhere.
A few years ago that wasn't really possible. Today, thanks to WebAssembly, browsers run code at close to desktop speed. Open-source libraries such as pdf.js (Firefox's PDF viewer), pdf-lib (which writes PDFs) and the Tesseract text-recognition engine run happily inside the browser. Yeti is built on them.
A note from Yeti: I don't come down from my mountain, and your files don't leave your device. That's our deal.
Test 1: Check the Network tab (the most reliable way)
On a computer:
- Open the PDF site you want to test in Chrome, Edge or Firefox.
- Press F12 (Cmd + Option + I on a Mac) and switch to the Network tab.
- Wait for the list to fill up, then clear it with the trash-can icon.
- Pick a file and start the operation. A file of a few megabytes makes the difference easier to see.
- Look at the new rows: the "Size" column and the request type.
If the file is being uploaded, you'll usually see a POST request about the size of your file. Click it and open "Payload", and you may even see your file name.
If the tool runs in the browser, you'll see either no requests at all or small ones without your file in them: fonts, parts of the program, a usage counter. None of them will be the size of your file.
Test 2: Airplane mode (the easiest way)
- Open the tool's page and let it load completely.
- Cut the connection: airplane mode on a phone, Wi-Fi off on a computer.
- Pick your file and start.
A tool that needs a server shows an error or waits forever. A tool that runs in the browser finishes and lets you download the result.
A small caveat: some tools download parts they need the first time you use them. OCR, for example, loads the language data when it starts. Run the test with something simple like merging or rotating first.
Is "we delete your files after an hour" good enough?
It's a serious commitment. The issue usually isn't bad intent, it's uncertainty. When you read that sentence, you can't know:
- Which country and which cloud provider the file is stored with
- Whether a copy ends up in error logs or backups
- Whether the service will be sold to another company one day
If you work with personal data, these questions aren't abstract. Rules such as GDPR, HIPAA or Turkey's KVKK expect a legal basis whenever you hand personal data to a third party. If the document never leaves your device, the question never comes up.
The limits of working in the browser
To be fair, this approach has its costs too:
- Speed depends on your device. OCR on a 300-page scan takes much longer on an old phone than on a powerful server.
- Memory is limited. Very large files can strain a browser tab, which is why Yeti caps files at 25 MB and operations at 200 pages.
- Some jobs need a server. Turning a web page into a PDF from its address means the page has to be fetched from somewhere. In Yeti, the only exception is the web-address mode of HTML to PDF, and the privacy page says so plainly.
Other ways to work on PDFs without uploading
| Option | Setup | Note |
|---|---|---|
| Browser-based tools (like Yeti) | None | Works on phones too; speed depends on the device |
| macOS Preview | Built into Macs | Merge, rotate, sign; limited conversion |
| LibreOffice Draw | Free program | PDF editing and PDF/A export |
| PDF24 Creator | Windows program | Free, lots of tools |
| Stirling-PDF | Self-hosted | Open source; good for teams |
Frequently asked questions
Is iLovePDF or Smallpdf safe?
They're established services that have run for years and say they delete files shortly after processing. They do, however, process files on their own servers, so your file sits on their infrastructure for a while. For everyday documents, that's fine for most people. For personal or confidential documents, a tool that never uploads the file is the more cautious choice.
Can Yeti see my files?
No. Your files are processed in your browser; their contents, names and passwords are never sent to Yeti. You can verify it yourself with the two tests above.
Can I merge PDFs offline?
Once the page has loaded, yes. Yeti's merge, split and rotate tools keep working after you go offline.
How do I share a PDF with personal data safely?
First remove what isn't needed with Redact PDF, which really deletes it, then strip metadata with Sanitize Document. Add a password with Protect PDF if needed.
In short
If the Network tab shows a POST request the size of your file, the tool is uploading it. A tool that still works after you cut the internet is doing the work on your device. Try it now: open Merge PDF, open the Network tab, merge two files and see what shows up in the list.