Create a PB from a .docx and build it.
Now move the .docx file to some other folder.
Rebuild the book.
You get an orange triangle with an exclamation mark in it, which if you hover over tells you "File not found" (about the missing .docx file).
Now copy the .docx file back to where it was.
Rebuild the book.
It builds successfully and you can open it, but the orange alert triangle error is still there. A successful build should clear that error state.