A screenshot essentially takes a snapshot of the visible screen area. As such, precisely what you see on the screen (and no more) will be captured by the screenshot - and can be saved as an image (to your Photo Library) or as a PDF (to files).
When using a web-browser, such as Safari, an individual web-page may be considerably longer than the viewable area of the screen - and consequently you must scroll up or down to read the entire page. It is when taking a screenshot of long web-page that the option to screenshot either the "Screen" or the "Full [web] Page" is available.
When viewing multi-page documents, or a large image that doesn't entirely fit within the viewable screen area, a screenshot will only capture the area of the document that you see on the screen.
With benefit of this knowledge, you hopefully now have a better understanding of what a screenshot - and how it can be used.
If you can outline precisely what it is that you are attempting to achieve in, presumably, capturing a screenshot that extends beyond the viewable area of the screen, there may be other methods to achieve your aim. By example, you might use the share-sheet (often accessed using the App's Share or Print button) to capture a PDF of an extended document.