If you are getting trouble to upload PDF file which size is larger then 10 MB on WordPress you should check your Apache server mod_security configurations. I added some steps about how to disable mod_security for Apache Web Server.
Step 1: Check mod_security File
#cat /etc/httpd/conf.d/mod_security.conf|egrep 'SecRuleEngine|SecRequestBodyAccess'
"SecRequestBodyAccess" and "SecRuleEngine" must be "On" if you have enabled mod_security functions.
Step 2: Disable mod_security
SecRuleEngine
SecRequestBodyAccess
Find these two parameters and change them to "Off"
#vi /etc/httpd/conf.d/mod_security.conf #SecRuleEngine On SecRuleEngine Off #SecRequestBodyAccess On SecRequestBodyAccess Off