In short
Yes. When you add a file, this site loads a PDF engine into your browser. The engine runs inside the browser's sandbox, nothing is installed on your computer, your files stay on your device, and every part of it is open source. You can check all of this yourself.
What the engine is
The engine is about 31 MB of files that your browser downloads from this site the first time you use it:
- Pyodide: the Python language, built as WebAssembly.
- PyMuPDF and MuPDF: a well-known PDF library, also built as WebAssembly.
- A small amount of our own Python code that does the compressing, rotating and so on.
WebAssembly is compact code that your browser runs inside the web page, the same way it runs JavaScript. The engine is not an app for Windows, macOS or your phone: nothing is installed. Your browser keeps the files in its normal cache so the next visit is quick, and clearing the site's data removes them. The site stores nothing else: no cookies and no saved data.
Why it cannot act like a virus
Browsers run web code in a sandbox: a closed space with strict limits. The PDF engine runs in a background part of this page and cannot:
- read or change files on your computer: it only gets a copy of the PDFs you choose;
- see your other tabs, other websites' data or your saved passwords;
- install software, start programs or change your settings;
- keep running after you close the tab.
These limits are enforced by your browser, not by us, so you do not have to take our word for them. They are the same limits that protect you on every website you visit.
How we make sure you get the right files
- Every engine file that comes from another project (Pyodide, PyMuPDF and MuPDF) is pinned to an exact SHA-256 fingerprint. If a single byte of one of them were different, our build would stop and nothing would be published. Our own code is in the source code download.
- All files, including the fonts, are served from this site itself. No other website is contacted while you use it, so no one else can swap them.
- Everything is open source. The source code of this app and the source of every part of the engine are listed on the licences page.
Check it yourself
- Nothing is uploaded. Open your browser's developer tools (F12 on most computers), go to the Network tab, and use any tool. No request carries your file.
- It works offline. Open a file and wait until the engine is ready, then turn off your internet connection. The tools keep working, which shows the work happens on your own device.
- Read the code. The source code link at the bottom of every page gives you all of it.
More
How this site handles your data is explained on the privacy page.