Is it possible to export to PDF (with the overlays) in landscape mode to fill the page ?
Yes. Instead of choosing File ▸ Export as PDF, first choose File ▸ Page Setup and select Landscape as Orientation. Then choose File ▸ Print, and at the bottom left select "Save as PDF". There are more options there if you select "Show Details", e.g. multiple items per sheet, enable borders.
Thanks Sander. Where can I find the legend description for the overlay symbols circle square reticle solid dashed color etc.
It was missing from the documentation, but I've just added some initial details to https://codeobsession.com/observatory/doc/#editor-overlay