I know it sounds obvious, but did you try deleting the files and re-downloading? Also, where did you download them to a computer or a mobile device like a phone? Because I have found sometimes when I download to my phone, PDFs could be blank when I open them in the file section of my phone, but if I open it in an application like Adobe PDF I can see them fine.

and what of it? The creator of the game is transparent about it.

How are things looking for Delve Darker Dungeons?