Signals Notebook: 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: to add an external hyperlink field. 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: to find and select internal references.

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.