Open Safari (or another app; see below) and take one screenshot of any screen, then immediately tap on the thumbnail preview. Above the screenshot will be two options: "Screen" and "Full Page".
Selecting "Full Page" will turn the entire document you are viewing into a PDF file that you can then save and manipulate in the Files app.
Note 1: The feature works in Safari, Pages, Numbers, Keynote, Mail, or Apple Maps. Note 2: Works on iPhone and iPad, iOS 13 and later.
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