I am creating a standards booklet with all of my offices standard details in PDF.  How can I make it so that once the detail is in PDF form all I have to do is click on the drawing in the PDF and it will open up in AutoCAD?  I know this is possible as my last firm did it but I cannot figure out how.

I'm guessing here, but the first thing I'd try would be to see if it's possible to set up a hyperlink in the PDF using Adobe Acrobat.

attache pdf the same way as you do with xref

I think what the OP is trying to do here is create a PDF that will open a specific DWG.  That work is outside the scope of AutoCAD.