@Doug.C You could do this similar to how IDC drops an xml file and pdf that are picked up with a import and added to ECM and attached to Epicor. I am actually try to figure out that import now. I know it reads the xml file and maps the fields, but it also grabs the pdf behind the scenes which appears in the workflow, but I do not know how that process works.
You could also just drop the pdf with a file name of where it needed to attach and have a simple workflow to parse the name and call attach to ERP.
The link below uses the AttachmentBO which has download and upload, but there is no company referenced.
This is the list of all Tasks available in ECM. Some of the task names are very clear about what it actually does so I have had to just click on a bunch to find out what they do. https://help.docstar.com/help/workflow-designer/action-editor/