Sumatra should have screenshot and with scrolling facility.
Screenshot is usually using print screen key =ALL screen(s) or Alt Print screen = ALL SumatraPDF window
Those are both Windows functions SumatraPDF does not need to replace them.
Inside SumatraPDF you can
Copy image (right click) it does not make any sense to scroll (but your zoom may have an effect)
Copy Selection Area = Ctrl & Left mouse to start then release Ctrl and keeping left button down select end . You can scroll at the same time as keeping the left button down.
Select start of text then Scroll up down etc. then middle click (or move to a white area) to complete the selection then right click to copy
Windows will only let SumatraPDF capture one or the other at a time IF you want image and text you can select area as image and text then save as image
If you selected text by scrolling between pages you will get both page parts plus the footer and heading between them (but not any embedded images. (that is a windows limitation)
Note although SumatraPDF will allow it, windows cannot capture an area whilst scrolling between multiple pages that is a clipboard image limitation. MS Paint etc. will only show the first one.
Due to the windows limitations if you want to capture page 2 or 3 etc you need to be in page mode and select all (right click) on each page and save each page in turn as image.
For just the plain text its easier to use save as text.
Quality especially the spaces between words etc. is down to the author in nearly every case. If the text is badly presented or garbled that is the source at fault.