This is probably worthy of a bug report.
PTC outsources the PDF creation to a 3rd party which created a DLL. This DLL was not available in 64-bit form until Wildfire 5.0, so even though you could export to PDF in 32-bit WF3 and WF4, you couldn't in 64-bit WF3 and WF4. So it made sense to remove the icon in 64-bit WF3 and WF4.
Now publishing PDF's is available in WF5, but it seems this icon is still not there. Hence, bug report.
Alternatively you can go to the "Publish" tab in the ribbon and export to PDF that way, or, as bianto has suggested, create a mapkey.