When you print to PDF from some programs, you end up with a PDF that looks like a printed image instead of text. In a PDF like that, you cannot select or copy text because the PDF reader cannot determine what is text and what is graphics.
This situation occurs when the printing program sends everything to the PDF printer as an image. The recent versions of popular browsers such as Firefox, Google Chrome, and Opera does this. There is nothing much you can do about this other than trying to use another program to print. Try using the Microsoft Edge browser or Internet Explorer. They both print the text as text so that you can select it and search it in the PDF.