Data URL of the rendered image (persisted for offline/cross-session)
OptionalpageFirst PDF page number (1-based). Kept for backward compatibility.
OptionalpageFull list of selected PDF page numbers (1-based). Lets the page-picker restore the user's selection when the same PDF is re-opened.
Source file ID from the file manager
Source file name for display
'image' for direct images, 'pdf-page' for rendered PDF pages
A background page — an image or rendered PDF page used as the canvas background.