Error Message: "You must change the sites/all/libraries/mpdf/graph_cache permissions to be writable, as mPDF requires write-access to that directory."
Actually, it looks like it's not writeable by the "apache" user. It's read only. It is writeable by the "hpadmin" user though. Currently "hpadmin" is the owner and "apache" is the group.

drwxr-xr-x.  2 hpadmin apache      22 Jul  2 10:51 graph_cache


vs

drwxrwxr-x.  2 hpadmin apache      22 Jul  2 10:51 graph_cache


You can adjust the permission to group writeable with 

chmod 775 sites/all/libraries/mpdf/graph_cache


or change "apache" to the owner with

chown apache:apache sites/all/libraries/mpdf/graph_cache