how do I convert a pages document into a PDF file

You open the Pages document and choose Export To -> PDF -> Choose name and destination of the written PDF — or you can select the Print… PDF -> Save as PDF sub-menu option, again choosing the name and destination of the PDF.

With the document open try File > Export to > PDF...

You can also (as you can on most Mac applications) File > Print and choose Save to PDF in the dropdown at the lower left.

SG