If the .fec-File-Format page isn't adequate, you will want to get the FEC's official documentation. As of this writing, you can download it directly from the FEC's AWS bucket:

• electronic filing official documentation
• paper filing official documentation

But if those links go bad, then you can also try manually:

1. Go to https://www.fec.gov/data/browse-data/?tab=bulk-data
2. Expand the "Electronically filed reports (.fec files)" section.
3. Click the download link.
4. Do the same for the "Paper filed reports (.fec files)" section.