You can Add an Object (Adobe Acrobat File) on a worksheet in the document (preferably very far down on the sheet, out of sight.
Wherever you want the link, insert hyperlink to... then select "place in this document"... then select the sheet the PDF file is located... then where it says, cell reference, enter the cell nearest the top of the document on the sheet where it is located.
this will create a hyperlink to the PDF object that is stored INSIDE the workbook you're sending.
**Note**
This makes for very large workbooks though because your PDF is stored inside the workbook. If you're hyperlinking more than 1 PDF, I recommend storing all of them on a single worksheet with "display as icon" checked to save space as mentioned above. Than they can doubleclick the one they need; bummer you can't hide the sheet though.