Error: unable to open a file (Invoice PDF)
The reason why you have the error below when downloading a PDF is the missing PDF notification settings set.
unable to open a file '/tmp/pdfname.pdf': -1
Error Code: 0x0abcac97.
Report ID: 1234567

Solution
Go to classic control panel by clicking in the right top corner
(Billing) > System > Settings
(System) > Notifications > Common Notification Settings
EDIT
Goto 'PDF Templates' and set these default values:
Invoice PDF Template: 'Invoice pdf notification'
CM PDF Template: 'CredeitMemo pdf notification'
DM PDF Template: 'Invoice pdf notification'