Take a look at this function:
Get ( DesktopPath)
It will reliably return the precise path to your client computer's desktop no matter from which computer (mac or windows) you evaluate the calculation. It will also adjust for different users as needed. You can then append just the text needed to complete the file path that you need by specifying the folders actually located on your desk top.
Value: Get ( DesktopPath ) & "Report Folder/"&"Report.pdf"]
If that fails, put a file in the folder where you want to save your PDF. Insert this file into a container field using the "by reference" option. Then use GetAsText container field, to get the file path to the file--which will be the last Row of text returned by this function. (You'll get three rows of text separated by returns.)
It may be a permissions issue. You have to make sure the directory/folder has read/write permission for fmuser when you are saving a pdf by script.