Okay I should have found out before that there are many good apps solving the problem. Thank you for your advices anyway! After testing some of them, let me als

Okay I should have found out before that there are many good apps solving the problem. Thank you for your advices anyway! After testing some of them, let me also answer my question by myself. I tried the apps Simple Scan, Tiny Scanner, Cam Scanner and Microsoft Office Lens. I skipped Adobe Scan, as I had to register before being able to use it. (The Free versions of) Simple Scan and Cam Scanner had too much ads for my taste. I could not find Readdle's Scanner Pro for Android in Google's playstore. Imo Tiny Scanner worked best for my particular problem. You can take picture after picture, cropping, stretching and brightness adjusting is almost automatic. After that you can save the scan series into a single PDF file. Also of good quality is Open Note Scanner, which however does only cropping and brightness/contrast adjustment without stretching, cropping and assembling to a PDF file. For photographs, paintings and screens I also liked Microsofts Lens, which is perfect for stretching and cropping without brightness/contrast and monochrome adjustment.