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Is it necessary for everyone to register in order to view or comment on a file?

No, registration is not mandatory.

To share a video, PDF, HTML or image with someone who is not registered, you get the option to use a Sharable Link and allow the person to either View the file or add Comments. As the file owner you can also give reviewers access and ‘Allow comment without login‘.

When you share the file with reviewers, they receive a link to the file via email. The link directly launches the proofing window that allows the reviewer to view or comment on the file without the need to login or register.