"The task could not be completed. The Device storage area may not be writable"

I have the Adobe Acrobat Reader for mobile on my Samsung Galaxy S5. I used to own a Samsung Galaxy S2 and was able to delete PDF files from the app itself. However, when I try and delete files from the app now, it pops up with a message saying, "This could could not be completed. The device storage area (eg. sdcard) may not be writable). I'm not sure why this is happening. For added context, the files that I am trying to delete are from a SD card and I can still delete files off from my phone's storage.