You can prevent your iPhone or iPad from auto-locking while using PDF Expert. It’s handy when you’re reading a book for a long time without performing any actions on the screen.
For now, there isn't such an option in PDF Expert for Mac. As a workaround, you can set sleep and wake settings for your Mac, and this will affect PDF Expert as well.
How to prevent the iPad from automatically sleeping:
How to prevent the iPhone from automatically sleeping: