In some cases, there's a chance that PDF Viewer won't be able to open any PDF Documents that you may have stored, either locally on your device, or on any of the cloud providers that we support. The documents, rather than showing a preview of their contents, would be greyed-out and show a new icon or graphic.
We have identified this as a bug that can be caused by third party applications modifying what iOS identifies as a "PDF document".
Here's a list of applications that we have identified as having caused problems for PDF Viewer customers before:
MarginNote (fixed in version 3.5.4)
Digital Paper App for mobile
To regain the ability to open documents in PDF Viewer, search your device for the application associated with the icon that's shown on the greyed-out documents. Once you've identified the application, you can
Make sure you're running the latest version of such application.
If the issue is not resolving by updating it to the latest version, remove it.
As a last resort, update to the latest version of iOS.
What we believe is happening, from a technical perspective, is that the system iOS uses to deliver files to document-based applications, such as PDF Viewer, is using a system-level API that returns a random UTI (uniform type identifier) for a given file extension, rather than using the API that returns all compatible file types for that extension.