PDF Expert for Mac supports several ways to open PDF files.
You may choose one option that suits your workflow best or use all of them.
To open a file in PDF Expert, tap the plus button at the bottom right corner and choose where to import a file from.
Also, it's possible to open files in PDF Expert through the other apps' share option. Open the needed file, tap the share icon and choose PDF Expert from the list. If there isn't PDF Expert, tap More and look for it in the expanded list.