How to convert and edit files without uploading them
Browsers can do most file jobs themselves. What runs locally, why it is enough, and which tools to use.
The default is upload, and it does not need to be
Search for any file chore and the first page of results is sites that take your file, process it on their server, and return a download with an ad and a limit. That model made sense in 2010, when browsers could not decode a HEIC photo or rewrite a PDF. They can now. JavaScript and WebAssembly let a web page run the same open-source libraries a desktop program would use, on your device, at desktop speed.
What runs locally
On Stayput, all of it. Specifically:
- HEIC to JPG or PNG: libheif, the decoder used by Linux desktops, compiled to WebAssembly.
- Image conversion, resizing and compression: the browser's own decoders and encoders through the canvas API; JPEG XL and AVIF via the Squoosh decoders.
- Removing EXIF and location data: a byte-level editor for JPG, PNG and WebP that leaves the image data unchanged (a sideways JPG is only re-encoded if you leave Apply orientation on).
- Merge, split, rotate, reorder, sign, page numbers and compress for PDFs: pdf-lib for editing and pdf.js, Firefox's PDF engine, for rendering.
- Images to PDF and PDF to images: the same two libraries.
Why local is enough
A laptop or phone from the last several years converts a hundred HEIC photos in under a minute and merges a 200-page PDF in a second. The upload to a server would take longer than the work. There is no quota because nothing costs the site money per file; there is no queue because there is no shared server. And the privacy property is not a promise: the file is never in a request, which you can check in the network tab.
What still needs a server
A few jobs do: OCR of long scanned documents (possible locally but slow), format conversions that depend on proprietary software (Word to PDF with perfect fidelity, some CAD formats), and anything involving another person's system, like an e-signature workflow. For those, choose a provider whose retention policy you have read, and send only what you would send by email.
Working offline after one visit, and installing it
Because every tool is a static page and a script, the site caches its pages and code once a page has loaded, and most tools keep working with the network off from then on; the few that download a model or engine need one online run first. In Chrome, Edge and Safari you can install it as an app from the browser menu; it takes no space worth mentioning and opens like any other program.
Questions
Is a browser tool slower than a desktop app?
For these jobs, not noticeably. WebAssembly runs the same code at close to native speed, and the work happens on your own processor.
Do I need to install anything?
No. Open the page. Optionally install it as an app from the browser menu; after one visit and a short wait, most tools then work offline, and the few that download a model or engine need one online run first.
What about very large files?
The limit is your device's memory. A few hundred megabytes of PDF or a few hundred photos are fine on a laptop; a phone manages less at once.
How do I know the code does what it says?
It is open source under the MIT licence. The repository is linked in the footer of every page, and the network tab shows what the page does while you use it.