Why are pdfs on my SD card becoming corrupted?

[AC Question]

I have a HTC one m8. I have recently installed a 128 GB SD card into it. I loaded the card up with epubs and pdfs. All was going well for a day or two. All of a sudden, with no apparent rhyme nor reason, I am unable to access pdfs within some of the folders. At first I couldn't open pdfs in one folder, but I could in another. Now I can't open files in either.
Depending on which application I try to open the files in, they are either invalid file types or corrupted.
What is causing the files to become corrupted and how do I stop it from happening?

 

[anon(782252)]

Hi and Welcome to AC!

Do the PDF files work fine if you put the SD card in a PC?

Maybe sectors of the card are starting to fail.