Thanks for your comments, everyone. I found the source of the issue. Only documents with major version levels were being displayed for Readers because I had set Document Library > Settings > Versioning Settings > Draft Item Security to 'Only users who can edit items'. I was mislead by the description to believe that this was the way to prevent non-authors from seeing document changes until they were checked in. Instead, it means that only documents with major versions (1.0, 2.0) are displayed. In SharePoint lingo, Draft = Minor Version.
