Write on a PDF
The form arrives as a PDF and you cannot type in it: everyone's shortcut is print, fill in by hand and scan again. Here you tap where you want to write, type, add tick marks where they belong and download the filled-in PDF. The file stays in your browser.
Tap the sheet where you want to write. You can then move the text with the handle and remove it with the ×.
A form that is not a form
There are two kinds of PDF that look the same: the ones with real fields (you click inside and type) and the ones that are just a picture of the sheet, which are the majority, especially when they come from an office or a scanner. On the second kind there is nothing to click, and that is where the print-pen-scanner loop comes from. This page writes on top of the PDF the text you dictate: what ends up in the file is real text, not a photograph, so it stays sharp and can even be searched.
How you write, and how you move it
Tap where you want to start and type: the point you tap is where the word starts, like putting a pen down on paper. Text can be moved by dragging the handle, deleted with the ×, and made bigger or smaller by picking the size. Move back and forth between pages and write where you need to: every piece of text stays on its own page. For boxes to tick there is the tick mark, the real PDF one, not a hand-made x.
Crooked pages, which are the real trouble
A scanned form is often rotated: the page is upright but inside the file it is turned by 90 degrees, and some programs show it straight anyway. Anyone writing on top without taking that into account puts the text sideways, or off the sheet, and only finds out after printing. Here the rotation is read from the file and the text lands where you put it, at 90, 180 and 270 degrees, and also when the sheet has a shifted origin (it happens: the corner of the sheet is not always zero).
What happens to your file
Nothing, it stays where it is: the PDF is opened in here by your browser and is never uploaded. What you download is a fresh copy with your writing added; the original is untouched. If the PDF is password protected the password has to come off first, and there is a page for that: Remove a PDF password.
What it does not do
It does not sign: for a hand-drawn signature there is Sign a PDF, which is used alongside this one. It does not fill the fields of PDFs that already have them (there your reader is in charge). It does not erase or cover what was already written: for that there is Redact a PDF, which really removes the text under the black. And it writes with the standard PDF font, which has every accented letter but no emoji: if you use one, it tells you instead of printing an empty box.