How to scan a document to PDF on Android
A photograph of a document is not a scan. The scanner your phone already has finds the paper, straightens it, and writes a PDF, and on most Android phones it is two taps away in an app you would not think to look in.
Why a photo of a document is not a scan
A photo taken over a page keeps everything the camera saw: the desk, the shadow of your own hand, and the fact that you were standing slightly to one side, so the page comes out as a trapezium rather than a rectangle. It is also a JPEG, which means several pages arrive as several separate images with no order to them.
A scan does three things a photo does not. It finds the edges of the paper and cuts the desk away, it corrects the angle so the page is square again, and it lifts the contrast so grey paper reads as white and pencil reads as black. Then it writes the result as a PDF, which is one file holding pages in order.
Option 1: the scanner already on your phone
This is the part most people miss. On the large majority of Android phones a full document scanner ships as part of Google Play services, and Google Drive puts a button on it.
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Open Google Drive.
Not the camera. The scanner is not in the camera app on most phones.
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Tap the + button, then Scan.
On many phones there is also a Drive scan shortcut you can drop on the home screen, which skips straight to the camera.
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Hold the phone over the page.
It looks for the edges on its own and shoots when it is happy. If the light is poor it will wait, so a lamp helps more than a steadier hand does.
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Add the next page, then save.
Keep photographing pages and they collect into one document in the order you took them. The save writes a single PDF.
Samsung phones have a second route. Point the built-in camera at a document and a "Scan document" prompt appears over the viewfinder. It is quick and it is good, though it hands you an image rather than a multi page PDF, so it suits one page rather than twenty.
Option 2: turn images you already have into a PDF
Scanning is no help when the pages arrived as photos from somebody else, or as screenshots. The job then is not scanning but assembly: several images in, one PDF out, in an order you choose.
Android has no built-in way to do that. The print dialog gets close, because Print to PDF will take an image and write a PDF of it, but it takes one image at a time and gives you no control over the order, so a ten page document means ten files you then have to join.
What decides the file size
A scan is photographs of paper, so the size follows the resolution and the colour. A twenty page colour scan at full resolution can run to tens of megabytes, which is what makes an email bounce.
- Black and white, or greyscale, for text. Printed text loses nothing and the file gets much smaller. Keep colour for anything where the colour carries meaning.
- Compress at the end, not while scanning. Scan at the quality you have, then compress a copy. Compressing first and finding the small print unreadable means scanning it all again.
- Check the small print before you send it. Open the PDF and zoom in on the smallest text on the page. That is the test that matters, and it takes a second.
Signing what you just scanned
The reason for scanning is often that somebody wants the document back with a signature on it. Printing it, signing it and scanning it again is the loop worth avoiding: a signature drawn with a finger and dropped onto the page is accepted for most everyday paperwork, and the page never leaves the phone.
Anything where the signature has legal weight is a different matter, and a scanned squiggle is not the same thing as a qualified electronic signature. For a delivery form, a school letter or a rental inventory, it is what everyone means.