"Free" file tools have a business model you never see: your file. Upload-based converters quietly pay for themselves with retention — copies on disk, CDN logs, "temp" storage that isn't.
We spent months auditing this space while building Toolfyra, and turned the audit into a checklist you can use on any tool:
The 5-question audit
- Does the file leave your device at all? DevTools → Network tab while converting. If a multipart upload fires, your "private" document is now their asset.
- What does the privacy policy say about retention? "Deleted after 2 hours" still means 2 hours of their custody — and policies change.
- Is there an account wall? Email + password for a unit converter is a data-harvesting tell.
- Where do errors go? Crash reports that include filenames are a quiet leak.
- Could this run locally instead? If the answer is yes (and for PDFs, audio, images it usually is), the upload is a choice, not a necessity.
The local alternative is real now
Modern browsers ship everything: crypto.subtle for hashes, Web Audio for sound, Canvas for images, wasm for the heavy stuff. Even breach-checking can be private — the k-anonymity pattern (send only the first 5 chars of a SHA-1, like our password checker does) proves you can query a leak database without revealing the password.
All 385 of our tools process on-device; the full index and every guide is in one sitemap: https://toolfyra.com/sitemap.xml
Disclosure: I run Toolfyra — but run the 5-question audit on us too. The Network tab doesn't lie.