Edits project.json - open it from Project Files → project.json. Sets up the page itself, before laying anything out in the Layout Editor.

Canvas source

• Use base.pdf - page size, page count, and background come from a real PDF file at the project root. Use Create base.pdf to generate a blank page at a chosen size/color, or replace base.pdf with your own externally-designed file (branding, letterhead, etc.).
• Generate from these properties - skip the physical PDF entirely. Set a canvas size (mm) and background color directly below, and the app synthesizes a blank page of that size at render time. base.pdf becomes optional.

Margins (generated-canvas mode only)

A purely visual guide in the Layout Editor - they don't clip or reposition anything.