Include existing PDF files into new TeX document

If you plan to insert an existing PDF into a new LaTeX document that you will compile to PDF, you can use \includegraphics for this. Although, be prepared to get tons of errors complaining about overfill hboxes and the like.
You can use this package that takes care:

\usepackage[final]{pdfpages}

In order to insert a PDF, use:

\includepdf{$filename}

See the external link for more options.