TexMaker - "File not found" when viewing as pdf

The "View PDF" button in TeXmaker only opens an already existing file, that has the same basename as your .tex file. Meaning, if you're writing in a file called MyReport.tex, it will look for a file called MyReport.pdf in the same folder as the .tex file, and open this in the specified viewer.

To create a PDF from the source, you have to compile the document. The easiest way to do this is usually to use pdflatex, which in TeXmaker is called with Tools --> pdfLaTeX (shortcut F6), after which you can view the PDF (shortcut F7). There are also buttons on the toolbar for both