If you are wondering where to find quality PGNs to train, the free Lichess studies are the best options on the internet.
Lichess has some very good courses on most common openings. And it is real simple.
Use your favorite search engine to look for "Lichess <study name>". For example "Lichess Ruy Lopez:
When you open the study, look for the share icon. It is either <> or <>. On a mobile device, it will way down near the bottom.
Once you click the Share icon, it will show you the button to download the Study. Click that and you have the complete Study PGN downloaded.
Notice the 3rd Share button below the board? Click on it. It will show you an option to donwload "Study PGN" (the second blue button). Click on it, and it will download the PGN file for you. Upload it to your favorite trainer (chessPGNTrainer ofcourse), and you are ready to master that study!
Use your browser search function to search for a study of your choice on lichess
Click through the search results to the study
The third button is the Share button. Click on it to reveal the blue buttons. The first of the button "Study PGN", will download the PGN to your downloads folder.
In Chess PGN trainer, select "Import Local Files..." and choose the file in the "Downloads" folder.