It doesn't support pdf files. The only printers that can do what you are trying to do have a section in CWIS to submit jobs in specified formats, pdf is one of those.
This printer does not have that option so anything not submitted by a driver that will convert the job will fail.
You can confirm this by converting the pdf test file to .ps or .pcl and it will succeed using the typical LPR command lpr -S 192.168.1.100 -P lp testfile.ps
This is why the 72xx can do it
And the 6180 cannot, it doesn't even have a submission section, also note the highlighted jobs. 3 jobs sent, 2 pdf's (SurprisedCat.pdf) and one ps file (SurprisedCat.ps), the ps succeeded, and even shows as a pdf in the status.
Just noticed you specified a 7120, not 7220. Makes no difference, reason is the same, screenshot still matches ;-)