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