A few things I would do to handle this differently....
I am assuming the employee is notified some how that they have been awarded a certificate? No? Yes?
1. You don't need an "advanced" service for this...just a regular backend generc service will do.
2. have your Adobe form be your certificate then filled in with the employee's data from the backend service.
3. the employee can then print/save the form as they wish.
If this is driven by HR (ie. award the employee)....
1. same thing....no "advanced" service needed.
2. merge employee data into your Adobe form (which is the certificate).
3. HR admin completes the "certification for employee".
4. now workflow could pick this up if additional approvals are needed or send to the employee.
5. employee is notified (via email) of the award/certificate and follows link in email or through portal or NWBC to the process. From there, like above, they can print/save the cert as needed.
Now, if you are using FPM and/or you absolutely have to have an attachment added, then you simply need to look at SAP's own code for how a user does that and just mimic it. I have done it in the past for processes where I need my form within the process to check for an attachment included and used SAP's own standard functions to "read" attachments. Same idea.
Good luck!