I finally called Advanced Tech support from my hosting provider (InMotionHosting.com).
They quickly diagnosed the problem as a ModSecurity issue.
ModSecurity was denying the action.
ModSecurity: Access denied with code 403 (phase 2). Pattern match
"http://[a-zA-Z0-9._]{1,}?/.{0,}?\\\\.pdf\\\\b[^\\\\n\\\\r]{0,}#" at
ARGS:jform[articletext]. [file
"/etc/apache2/conf.d/modsec_vendor_configs/comodo_apache/09_Global_Other.conf"]
[line "21"] [id "211050"] [rev "1"] [msg "COMODO WAF: Universal PDF XSS
URL Detected.||www.southark.edu|F|2"] [severity "CRITICAL"] [tag "CWAF"]
[tag "Other"] [hostname "
www.southark.edu"] [uri
"/student-services/the-learning-center-tlc"] [unique_id
"WleSETi@87PXTm@Ppb@28gAAAFE"], referer:
http://www.southark.edu/student-service ... VyLXRsYw==