One of the main reasons why that happen is because a sub-par PDF creator was used to generate them. Sometimes overuse of a PDF file causes corruption, mostly due to software glitches.

Some email providers encode PDF attachments for faster transmission, causing them to become corrupted. This is by far the most common way PDFs become corrupt. When they are received by the recipient, they aren't recognized as PDFs or become corrupted. That's why it's recommended to send PDFs as a zipped file or to host them on the web (and send a link to the hosted file instead).

PDFs can become corrupt when uploading them to the internet via FTP. If the files are being uploaded via FTP, make sure you use the option for binary files and not ascii files. If you use a well-known sharing service like Google Drive or Dropbox, this happens automatically.

PDF files can become corrupt when copied. While the reasons for this occurring are not clear, the original file (that was copied) should not be corrupted. Try making another copy.

PDF files may also corrupt if you view them online directly instead of downloading them first. Many users report that their PDF files open very slowly or don’t open at all after they view them multiple times online. Opening a PDF file on your browser instead of a dedicated reader may also result in the corruption of the file.

Other possible causes of PDF file corruption include connection failures while downloading one (which results in a partial download), HDD failure, power outages, archive failures, system failures, making changes to it, printing it and virus attacks.