Before the official 2d20 system, several fan-made "Fallout PnP" systems were created:
A collection of lore and early design documents for a Fallout 3 project (Van Buren) that includes early pen-and-paper testing notes, often hosted as a PDF on sites like GOG. Fallout: The Roleplaying Game Downloads download-fallout-the-games-download-zip
The modern official tabletop game, by Modiphius Entertainment, offers several free digital resources: Before the official 2d20 system, several fan-made "Fallout
Various versions (like 1.1 or 2.0) of these community-made rulebooks can be found on document-sharing sites like Scribd or Internet Archive . Before the official 2d20 system
A free 64-page PDF on Modiphius containing basic rules and a starter adventure.