There's a bug in Proton Docs: sharing a Proton Doc with view-only permissions between Proton IDs (direct email invitation, NOT via public link) shows a blank page to the recipient. The only way to view the document is to open its history. This affects both free and paid Proton accounts equally.

Additional Details:

Browsers tested: Firefox, Vivaldi, Edge, Chrome, Mulvard — same blank page result in all
Accounts tested: Multiple Proton accounts (3+ different sender and recipient accounts) — consistent issue
Reproduction steps: Switching sharing permissions from Editor to Viewer in all instances produces the same result
Mobile (Android): Same blank page issue occurs on mobile
Critical finding: On mobile, when opening File Details for a document shared with view-only permissions, it still displays 'Permission: Editor' instead of 'Permission: Viewer' — suggesting the permission change may not be properly applied or recorded

Workaround: Opening the document history allows viewing the content despite the blank page issue.