Is it possible within admin defined tables to have a file attachment field, like the one within the Samples table?

Please refer to our System Configuration Guide > Table Templates for available property types. Those include:

• External hyperlink: Allows the user to add an external link. Please note that in addition to HTTP and HTTPS, the hyperlink attribute field supports ftp, ftps, and sftp in the link field section.
• Internal Reference: Allows the user to add a link to an entity within Signals Notebook.

You can use Internal Reference to link to an existing attachment by pointing to that file (e.g. PDF) that is already in Signals Notebook. Or use External hyperlink if you prefer to reference a file that is outside Signals Notebook.