A search function to search all text on all pages (visible and not visible)

Site URL: https://aldricharchive.squarespace.com/

hi everyone, and in particular @tuanphan and @Agha_Waqas (the legends of this forum) 

I have a website which is an archive of text, files and images. I would like to be able to search EVERYTHING (all text on all visible and non visible pages) via a search box. I would like to type in a keyword and click search, then a page appear with the word highlighted/identified (if this is too much, just to bring up a page). I currently have button at the top right which I would like to be the search function (see screenshot) 

Is there a way to do this through standard functionality, or does this require some code? 

Thanks in advance

Pete