Try this approach:
Set Variable [ $Path ; Value: "file:" & Get ( DesktopPath ) & YourTable::FileNameField & ".PDF"]
Save Records as PDF [Restore; no dialog ; "$Path" ; //specify the other options you need for your script here]
In the save as PDF file reference, just use $Path, nothing else.
What you have computes an invalid file path as you refer to the desktop directory explicitly in the file reference box, but Get ( DesktopPath ) is also used to put it in your variable. Thus, the path and file name used comes out something like:
Where the text in blue comes from your $packetName variable.