Flux doesn't directly generate a PDF of your PCB design, but you have a couple of options:
Use Your Browser's Print Function:
Switch to the PCB layout view in Flux.
Use your browser's print option (usually via File → Print or Ctrl+P).
In the print dialog, choose “Save as PDF.” This will capture your current PCB view.
Export Manufacturing Files:
You can export your design as Gerber files, which are the industry standard for fabrication, and then use a separate tool to compile these into a PDF if needed.
Let me know if you need more detailed instructions or further assistance!