Is this document stored in a container field? Actual file or store by reference? One such document is always used?
There are ways to put the filepath of the document you want to attach in a variable and then you refer to the variable when you specify your attached file in your scripted send mail step.
The details depend in how you link your document to your current record in your database.
The files are PDF format and placed inside a container field.
If they are physically stored in the container (as opposed to just the reference), you can use export field contents and a variable to attach the PDF to an email.
Use set Variable to compute a file path for the file you are going to export and store it in the variable. You could use something like this:
Set Variable [$FilePath ; Value: Get ( TemporaryPath ) & "EmailedFile.PDF" ]
You can then use this path in the file reference for exporting the field and also as the file reference for attaching the exported file to your email in your Send Mail step.
You could use the same approach with store by reference files, but it seems simpler just to extract the file path from the container field in those cases and then put that path in the variable instead of exporting it.