Unexpected Server Response error when uploading password protected PDF to Word Press Media Library

Hello,

I need to have password protected PDF to post on internet.  But, when I try to upload the password protected PDF into my Word Press site -- Media Libray, add new -- I get the error message "Unexpected response from the server.  The file may have been uploaded successfuly. Check in the Media Library or reload the page" -- then the PDF (after Media Library page reloaded, appears as an icon (non-descript) instead of as a thumbnail (with picture of first page of PDF). How do I upload password protected PDF into Media Library without getting error message, and how to allow the PDF to appear as a thumbnail in Media Library?