What is your question? How do you determine where they want their PDF exported? If so you can have an text box on your form that generates the report that asks for the path they want to put the PDF (there are some fancy things you can do like open a dialog box to let them navigate there, but if you're a novice it might be difficult to get it working). If you're talking about HOW to do it There's a function in access 2007 and higher to handle outputting to a pdf (syntax below)
Code:
DoCmd.OutputTo acOutputReport,"ReportName",acFormatPDF,"D:\Access\Reportname.pdf"