My Kindle suddenly stopped recognizing .pdf files. The ones I have copied to it before still are there and can be read - however, when I try to copy new ones they simply do not appear on the device after the process. Other formats work just fine (tested .mobi and .txt), but PDF just do not want to work. I have tried rebooting the Kindle, but to no avail. Anyone ever had this issue, or knows how to fix it? :/