I scanned an A4 document. Droid Scan correctly auto-sensed the document size, but when I generated a PDF of the scan, either the PDF was not A4, or the scan did not fill the page. What should I do?
First, to automatically generate a PDF that is the same paper size as your scan, choose "Match scan" for the PDF paper size in the preferences. This will work for any of the documents sizes that are also standard PDF page sizes (currently US Letter, US Legal, A4, and ANSI B).
Second, to ensure that, for example, an A4 scan entirely fills the page when printed as an A4 PDF, create the scan with a sufficiently high resolution ("High" or "Ultra"). If the scan is low res it will be automatically scaled down to meet a minimum print quality constraint based on dots per inch.