1 - A network connection problem between the user's computers and our servers than can lead to a bigger lattency when accessing the files, and
2 - Problem on the uploaded files. In 99% of the cases this behavior is due to misconfigured uploaded pdf files. Note that there are many different systems for creating pdf files and those pdf files can indeed be generated from another corrupted files, images, scanned documents, unrecognized formats. This way is practically impossible to say what corrupts these pdf files. In this case, each file has to be regenerated using recent programs/plugins to create pdf files.