https://github.com/OpenBoardView/OpenBoardView
The original open source boardview software
OpenSource, code available to everyone
Cross platform, including older OS versions
Github issues support
https://pldaniels.com/flexbv5
Modern up to date boardview software
Free, Demo, and paid licence version
Supports many file formats
Windows, macOS and Linux
PDF/Schematic support
OpenBoardData support
Closed Source