Compressing a payslip, merging a rental application, reading the text of an ID: these documents hold very personal data, and most online services ask you to send them to their servers to be processed. There is another way, where the work is done on your own device. This guide explains the difference, how to check it yourself and what it costs.
What happens when you upload a file
With a conventional service, your document crosses the internet, lands on a server you do not control, is processed there, then sent back to you. In the meantime it may be kept for a few minutes, a few days or longer, copied into backups, logged, or examined to improve the service, depending on the site's policy, which almost nobody reads. None of these steps is necessarily malicious, but each adds a risk: a data leak, a configuration error, a dishonest employee, a change of the service's owner, hosting in a country whose laws differ from yours.
For a document containing personal data (identity, health, finances), sending the file to a third party is already a data transfer. If you process other people's files, say in an office or a non-profit, you are also responsible for them.
How a browser can do it all alone
Modern browsers offer capabilities that often go unnoticed: reading a file chosen by the user without sending it anywhere (File API), running compiled code at nearly native speed (WebAssembly), working in the background without freezing the page (Web Workers), computing cryptographic fingerprints (Web Crypto). With these building blocks, a site can merge PDFs, compress images or recognise text entirely on your device. The program's code is downloaded like an ordinary web page; your files do not leave.
Comparison
| With upload to a server | Local processing (MaxyTools) | |
|---|---|---|
| Where your file goes | To the site's server | Nowhere: it stays on your device |
| Retention | Per the site's policy | None: nothing is stored |
| Offline | Impossible | Possible once the page has loaded |
| Speed | Depends on your connection and the server | Depends on your device's power |
| File size | Often limited by upload | Limited by the browser's memory |
| Daily limits | Frequent (accounts, subscriptions) | None imposed by a server |
Checking for yourself: three tests
- The Network tab. Open your browser's developer tools (F12 key), “Network” tab, then process a file. You will see the site's requests pass: if the file were uploaded, a large request would go to a server. In a local tool, nothing of the kind.
- Offline mode. Load the tool's page, then cut your connection (airplane mode). If the operation still works, the job does not need a server.
- The privacy policy. Read it: it should state clearly what is sent, to whom and how long it is kept. A site promising “deleted after one hour” asks you to trust it; a local tool has nothing to delete.
The first test is the most telling. You do not need to understand the code: simply look for whether a request the size of your file leaves during processing.
The price of local processing
- Memory. The browser has only part of the device's memory: on an entry-level phone, a PDF of several hundred MB can fail. Each tool states its maximum size (for example 80 MB for PDF compression, 100 MB for an image).
- Speed. Recognising text or compressing a large PDF on a modest computer is slower than on a powerful server.
- Battery. Heavy processing taxes your device's processor.
- Impossible features. Some processing (the heaviest artificial intelligence) remains out of reach of a browser; we prefer not to offer it rather than send your files elsewhere.
Other good habits
- Remove hidden data before sharing a file: photos and documents contain metadata (see Hidden metadata: what your files reveal and how to erase it).
- Protect what must be protected with a password (see Protect a PDF with a password, or unlock it).
- Be wary of sites demanding an account or an email for a simple operation.
- Delete downloaded copies you no longer need.
Honest limits
- Local processing protects the file during the operation, not your device: if your computer is infected, the file is exposed whatever the site.
- You must believe us on the basis of what you can verify, hence the importance of the tests above rather than our word.
- External components (advertising after consent, audience measurement) exist on the site: they do not receive your files, but read the cookie policy for details.
Frequently asked questions
How can I be certain a file is not uploaded?
Open the developer tools' Network tab (F12) during processing and look for a request the size of the file. You can also cut the connection once the page has loaded: if the tool still works, it works locally.
Does local processing work offline?
Once the page and its components have loaded, yes for most tools. The exceptions (breach check, IP address) need the internet.
Why are some files refused for size?
Because the browser has less memory than a server. The limit is shown on each tool.
Does the site still receive something?
It serves the page and its programs. The files you process are not transmitted to it. The privacy policy details what is measured.
Is it safer than installed software?
It is comparable for privacy: in both cases the file stays with you. The advantage is installing nothing.