How browser-based file tools work, and why your files stay on your device
Many online converters upload your file to a server. Tools that run in the browser do the work on your own device instead. Here is how that works and how to tell the difference.
Two kinds of online tool
Upload-based tools send your file to a server, process it there and send back the result. They can handle heavy jobs, but your file leaves your device, travels over the internet and sits on someone else's computer for a while, under their retention policy.
In-browser tools download a small program (JavaScript and WebAssembly) into the web page. When you choose a file, the browser reads it from your disk into memory, the program processes it on your processor, and the result is offered as a download. The file is never sent anywhere.
What modern browsers can do locally
Browsers now include fast image decoders and encoders, a canvas for drawing and resizing, and WebAssembly, which runs compiled code at close to native speed. That is enough for compressing and resizing images, converting formats, building PDFs, reading Word files, and even running OCR with an engine like Tesseract. Every tool on Paul Digital works this way, which is why they keep working if your connection drops after the page has loaded.
Why it matters
- Privacy: ID cards, certificates, contracts and family photos are not copied to a third-party server.
- Speed: there is no waiting for a large upload and download, which helps on slow mobile data.
- No account: there is nothing to store, so there is no need to sign up.
- Limits: processing uses your device's memory, so very large batches can be slow on an older phone.
How to check a tool yourself
- Load the tool page, then turn on airplane mode or disconnect Wi-Fi.
- Process a test file. If it still works, the work is happening locally.
- For a closer look on a computer, open the browser's developer tools (F12), go to the Network tab, and watch for requests while you process a file. A local tool makes no large outgoing request carrying your file.
Also read the privacy policy. It should say plainly whether files are uploaded and, if so, how long they are kept. Paul Digital's privacy policy explains what the website does and does not collect.
What still leaves your device
Even a local tool loads its page from a web server, so the server sees an ordinary page request. If the site uses analytics or advertising, those services receive standard information about the visit, such as the page address and your browser type, according to their own policies and your consent choices. Your files and their contents are a separate matter, and with an in-browser tool they are not part of any of that.
Frequently asked questions
Are Paul Digital tools free to use without uploading files?
Yes. The tools run in your browser and your files are processed on your device, not uploaded.
Why is a browser tool slower on my phone?
The work uses your phone's processor and memory. Large images or many files at once take longer on lower-end devices.
More guides
- What photo metadata (EXIF) reveals, and how to remove location before sharing
- How to compress an image to an exact size in KB
- Photo and signature size for online application forms: a checklist
- JPG, PNG, WebP, AVIF or HEIC: which image format should you use?
- All guides
Paul Digital guides are written from hands-on testing of the tools on this site and are reviewed when platforms or formats change. Requirements set by exam boards, governments and apps vary and change over time, so always check the official instructions for your case. Spotted an error? Email askpaulonline@gmail.com.