The solution is simple, to keep the aspect ratio when scanning with WinScan2PDF !
Start WinScan2PDF and before start the scan, enable the option Keep Aspect Ratio (... see Image-1 Arrow-1)
This option is available from version 2.77 in WinScan2PDF.
From Version 3.77:
(... see Image-1) Fit to the Page
use the source for the size of the PDF page! (... see Image-2)
Please use ... always the latest version of WinScan2PDF.
FAQ 4: Updated on: 1 July 2019 16:40