Invoice - problem with PDF attachment

Invoice - problem with PDF attachment

Guys - have you seen this?
 
I have been using 'Invoice' to email my invoice PDF directly from within zoho to an agency who then bulk process all their incoming email attachments (many hundreds). But, according to them, just my PDF causes a problem as it automatically prompts "do you want to print this?" when it's opened - which halts their entire batch processing.
 
I can't replicate the problem as the locally exported PDF opens ok on my PC. If I use zoho to send an email with the PDF to my email account, it also opens without issue. Maybe their application API is faulting on a 'flag' buried within the zoho auto-generated PDF.
 
I would appreciate your comments.
 
Regards
 
Martin.