Hyperref package

The objective is to obtain a pdf with bookmarks for each section and subsection.

Solution: 1) add this line \usepackage{hyperref}
2) above each section and subsection that you want a bookmark, type \maketitle
End results: The bookmark will be automatically generated. In addition, any place you use \ref or \cite in Latex, in the pdf file, a red box will appear.

This trick makes a pdf file have a more professional look.

For more info on hyperref, check this link.