How to scan a document without installing an app
Open a scanning tool in your phone's browser, photograph the page, and let it find the edges and straighten it. No install, no account, and on a tool that works client-side the photograph never leaves the phone. The whole job takes about thirty seconds.
Scan to PDF does exactly this in the browser you are reading in. Scan to PDF
Why you might not want the app
Document scanners are among the most aggressive apps on either store: several of the popular ones ask for contacts, location and full storage access, show ads between every screen, and put a watermark on the free export. For scanning one form once, that is a great deal of permission to hand over.
What the browser can genuinely do
Take a photograph, find the four corners of the page, correct the perspective so it looks scanned rather than photographed, lift the contrast so paper reads white, and write a multi-page PDF. All of that is ordinary work for a modern browser and needs no server.
What it cannot do
Read the text. Recognising words on the page is a different job, and on the web version it is not there — the PDF holds pictures of your pages. If you need the text searchable, that is text recognition, and it is in the Android app.
Getting a result that does not look like a phone photo
Put the page on something that is not the same colour as it, stand over it rather than leaning across it, and keep your own shadow off it. Those three things matter more than any setting.