This fault usually manifests itself when members who are not Super Admins try to upload a PDF using File Manager. This can also be the case when using the Wygwam fieldtype which uses EE's file upload system.

In these cases, the user will get the following message:

"File cannot be Written to Disk"

Try navigating to Admin -> Security and Privacy -> Security and Sessions and set "Apply XSS Filtering" to "No".

If that fixes it then you have a temporary but not an ideal solution. XSS (or Cross Site Scripting) is a security vulnerability and what this fix does is diable EE's security measures. There's a good post on Stack Exchange about a possible, more permanent solition.