The PDF scanned document are "pictures", so you can't select a line or part.
You could try to use an OCR like the good ABBYY Finereader.
I don't know what you understand when you said "important to me is size and resolution" : little or big screen ? If you are looking for big screen : ONYX M92 (
https://wiki.mobileread.com/wiki/Boox_M92), you can go to the forum :
https://www.mobileread.com/forums/forumdisplay.php?f=220
You can compare with a Pocketbook.
You can put some examples of files and ask for a particular test.