Hello maladwan
Yes, it fully covers the Bernina line. The Saver Day Pass is valid on all PE trains. See the validity map here https://www.sbb.ch/en/tickets-offers/tickets/day-passes/saver-day-pass.html → Downloads → Synoptic map.
When you would like to make a seat reservation, select that you already have a ticket and only want to make a seat reservation, then choose “Saver Day Pass” (or “GA Travel Card” if the Saver Day Pass is not listed) when you are asked what ticket you hold.
→ Keep in mind that the Saver Day Pass requires in addition your passport or ID card (no copies / no photos of it) upon ticket check.
Kind regards,
Roland