You may want to look into using the Generate Attachment Match Table, This would allow you to set up the way the attachments are matched.
Hope that this helps.
Attachments have been designed to use OBJECTID, so that cannot be edited/modified.
You can simply use a relationship class to relate the Table (containing a raster field) to the Feature Class. Then you can choose whichever key you want.
Note: Always try to provide your configuration details(Desktop version, DBMS version,etc) for a more appropriate answer/suggestions/links.
No, what I meant was--> Instead of enabling Attachments on the data....create the Relationship Class and the table manually, therefore choosing your Unique field in the process.
You don't enable Attachments in this workflow at any point of time. You will have to create a Table first, which will have a Raster field and then create a Relationship Class as per your requirement.
I knew this works for Images, however I just noticed that you are planning to attach PDFs. This workflow won't support PDFs, it only allows Raster datasets(images). Sorry for missing this previously.
I also just checked this Forum post:
I think Hyperlinks would be your way ahead...