I suppose you could persist it as a byte, jBPM will persist them, but there will likely be issues with a blob that big (talking on pure assumption here).
The alternative is to store some reference to your document in the process, and reference it when you need to with a link or some such assuming you have some PK to the external system (or your system) where the document is actually stored.
jBPM comes with a module definition called FileDefinition. This can be used to attach multiple files to a process.
Can you please put some light on FileDefinition. I am using jbpm-console application. Can I attach a document through that.