All Paper docs have specific access permissions: can comment or can edit. These permissions require that you authenticate with a Dropbox account before accessing Paper.
If someone shares a Paper doc with you, you may be able to view it without signing in. However, you can't interact with it (comment or edit) unless you sign in to Dropbox.
If the doc sharing permissions are set to Only people added or the doc is from a team where external sharing is disabled, you need to authenticate into Dropbox to view the doc. If you don't have a Dropbox account, you see the option to create one before viewing a Paper doc.