The most recent upgrade to app version 3.0 may result in your imported PDF files disappearing from your Library on the app. To recover your PDF files:
Ensure you are logged into your account on the app, under the Account tab.
Tap on the Import button along the left-hand side of the app.
Locate where your PDF(s) are stored; these will likely be found in the “On My iPad” folder, within a sub-folder called “Sheet Music”.
Tap on the PDF(s) you wish to select, then tap Open.
Your selected PDF(s) have now been imported, and will now be located within the Library tab.