I guess you want to save a long screenshot in .pdf format? You can’t do this directly (at least not by default, there might be apps for it), instead just take your long screenshot as usual and open it inside your prefered gallery app, then take the following steps:
- Select share
- Select print
- On the very top of the screen, select the printer
- You’ll see a “Print as pdf”-option right away, tap it and you should be able to save it in .pdf format
