Editing metadata in the GUI does not make changes TO THE BOOK IN THE LIBRARY. There are a number of additional steps to FORCE those to be updated DEPENDING ON FORMAT.
Transferring a book USING Calibre updates Metadata and Cover in the copy sent.
IMHO PDF is not a general purpose e-book format and I only use it when EPUB or AZW3 will not render the (usually, Technical or Artistic) subject matter well.
I also rarely rely on the books contained metadata once delivered to my device. Metadata helps me import and Select books. For that I use Calibre Library view

or Calibre Companion (on Android)