Paperwork has a habit of happening where the signal doesn't
Think about where you actually end up filling in forms. A clinic waiting room in a basement. The back office of a government building. A plane, an hour before you land and need the arrival form done. A train through a valley. Abroad, with data roaming off and the hotel wifi asking for a room number you don't have yet.
A tool that uploads your document can't help you in any of those places, and it tends to fail at the worst moment: not when you open it, but three fields from the end, when it tries to send. Doing the work on your own device removes that entire category of problem. There is no request to fail, so a bad signal is just a bad signal rather than lost work.
Install it like a regular app
Because every tool already runs locally with nothing uploaded, your device can simply keep a copy of the whole thing. Installing takes a few seconds and one connection, after which it opens from your dock or home screen like anything else you've installed.
Click the install icon (a screen with a down arrow) at the right-hand end of the address bar, or open the browser menu and choose "Install PDkef".
Tap the Share button, scroll down the list, and select "Add to Home Screen". It has to be Safari; Chrome on iOS can't do this.
Click the Share button in the toolbar and choose "Add to Dock...".
No install icon in the address bar? It usually means the page hasn't finished loading yet, or you're in a private window, where browsers don't offer it. You don't strictly need to install anything either: an ordinary browser tab keeps working offline too once you've visited once. Installing just makes it easy to find when there's no connection to search with.
Your half-finished form isn't riding on one open tab
Long forms rarely get done in one sitting. As you type into fields, tick boxes and place a signature, the tool writes your progress to your device's own storage in the background. Close the tab, run out of battery, or let the browser quietly discard the page while you take a call: open the tool again and your work is where you left it.
The document itself is saved alongside the edits, so you don't have to find the original file again either. Drafts are kept for 14 days and then cleared, and loading a different file into the same tool replaces the one that was there. Downloading your finished PDF deliberately doesn't wipe it, so noticing a typo two minutes later isn't a disaster.
The honest limit: this is your device's storage, not an account, so the draft stays in the browser you started in. Nothing syncs to a phone, because syncing would mean uploading, and that's the one thing this is built not to do. Crash recovery without a server is a fair trade for that.
Not a reduced offline mode
Plenty of apps have an offline mode that queues your work up and syncs it later, which means the important part still happens on a server eventually. This isn't that. The code that reads your PDF, places your signature and writes the finished file has always run on your device, connection or not, so there's nothing to queue and nothing waiting to be sent when you reconnect. Offline isn't a fallback mode here, it's just the normal mode with the wifi off. You can check that claim yourself in about a minute.