How can i screencap the entire page?

i work a lot on the internet and a hell of a lot of money for my age, i often go and look at other peoples websites and links and screencap their content to study later. I find it a pain in the *** the scroll down 3 or 4 times to get the whole page, how can I capture the entire page as an image or .jpg?
Update: for me, sysrq does exactly the same thing as prtsc. Only captures the screen, not the whole page on my browser.
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