With browsers, mouser actually recommends using a browser add-on to capture large web pages.
This example is different again in that it's a PDF within the browser - presumably in Adobe Reader?
see the first minute of
https://www.donation...c/3ds/sc21/sc21.htmlIn FF 10.0.2 I was able to get a good screenshot - but not easily:
I tested window & it couldnt scroll - recommended I click in the window & retest - that worked. I started the capture,
in the next dialogue, I clicked:
1. "START HERE: guess margins"
and
2. Guess overlaps... #2 gave me this message:
[Window Title] Screenshot Captor
[Content] ATTENTION:
The vertical overlap value could not be guessed. This means that either that the page
contents have no overlap, or, more likely, that the overlap was too small to
reliably detect or that the contents changed while scrolling.
Please check the stitched image and manually adjust the margin and overlap values.
Tip: After changing margins you might try pressing the "2" button to re-guess overlaps.
[OK]
I then had to change the Vertical value under "Trim & Show" (top-left in screenshot below)

I've had that experience before - and this was the last value I tried changing (it isnt really covered by the suggestion in the message above to change
"margin and overlap values" - to be honest I'm not sure what it is or does...). So, this time it was the first value I tried changing ;-)