Short answer
For a holiday photo or a public brochure, a reputable online converter is fine. For contracts, payslips, medical records, client files or anything you are not allowed to share, do not upload it: use a tool that converts on your own device. Here is how our browser-based tools do that.
Every "free online converter" that asks you to upload a file is, by definition, copying that file to someone else's computer. Most services handle this responsibly, but the copy still exists, on infrastructure you do not control, under a privacy policy you probably did not read. This guide explains what actually happens to an uploaded file, which documents should never be uploaded, how browser-based conversion avoids the problem, and how to check any tool's claims for yourself.
What happens when you upload a file to an online converter
It is copied to a server
Your file travels to the service's servers (often on a large cloud provider) and is converted there. Typical policies promise deletion after anywhere from one to 24 hours; some let you delete immediately. Until then, a copy exists outside your control.
Requests are logged
Like any web service, converters log requests — usually your IP address, file name, size and type, and the time. File names alone can be revealing ("Smith_divorce_settlement.pdf").
Policies vary
Retention periods, whether staff can access files, which subprocessors are involved and which country the servers are in all differ between services. Few people check before uploading.
Breaches happen
A file that has been deleted cannot leak. A file sitting on a server for two hours can, if that server is compromised during that window.
Files you should not upload
Business and client documents (GDPR)
Under GDPR, sending personal data about customers or employees to a third-party service makes that service a data processor, which requires a processing agreement (Article 28). Pasting a client list into a random converter does not meet that bar. Many companies forbid it outright in their IT policy.
Medical records (HIPAA)
In the US, protected health information may only be shared with vendors that have signed a Business Associate Agreement. Free online converters generally have not. Converting on the device avoids disclosing PHI to anyone — though your organisation's own HIPAA obligations (device security, access control) still apply.
Financial and identity documents
Bank statements, tax returns, payslips, passports and ID scans are exactly what identity thieves look for.
Journalism and sources
For reporters, the upload log itself can be the problem: it ties a document to an IP address and a time. Converting locally leaves no third-party record.
Personal photos
Photos often carry EXIF metadata including GPS coordinates. Uploading them hands over where they were taken as well as what they show.
How browser-based conversion avoids the upload
The file never leaves the tab
When you choose a file, the browser's File API reads it into the page's memory. Nothing is sent over the network unless the page's code explicitly sends it — and a no-upload tool simply does not.
Conversion runs as code in your browser
Images are decoded and re-encoded with the Canvas API or WebAssembly codecs. PDFs are rebuilt with JavaScript libraries such as pdf-lib and PDF.js. Video and audio run through ffmpeg compiled to WebAssembly. The same engines that power server-side converters, running on your machine instead.
The result is created locally
The converted file is assembled in memory and handed to you as a download. Close the tab and it is gone.
Sandboxed by the browser
Web pages run inside the browser's sandbox: they cannot read other files on your disk or install anything. You only grant access to the specific file you pick.
How to check whether a tool really does not upload
1
The offline test
Load the tool's page, then switch off Wi-Fi or unplug the network. If converting still works, the file cannot have been uploaded.
2
Watch the network tab
Open your browser's developer tools (F12) → Network, then convert a file. An uploading tool shows a large outgoing request roughly the size of your file. A local tool shows none.
3
Read the privacy page
Look for a plain statement of what is and is not collected. Ours is at /privacy: page analytics, yes; your files, never.
4
Strip metadata before sharing
Even after converting locally, the output can carry metadata. Use the EXIF remover on photos before posting them.
When uploading is fine — and when it is the better choice
Non-sensitive files
A public flyer, a meme, a screenshot of a web page: the privacy cost is negligible, and an established online service is perfectly reasonable.
Very large or heavy jobs
Server-side services can be faster for multi-gigabyte video on an old laptop, and some offer stronger compression for scanned PDFs than browser tools. If the file is not sensitive, that trade-off can be worth it.
Everything else
If you would hesitate to email the file to a stranger, do not upload it to one. Convert it on your device.
Convert files without uploading them
PDF, image, video and audio tools that run entirely in your browser. No upload, no account.