I am trying to up load a pdf that is quite sizable. It is 15 mb. I have increased the allowable limit for uploads with no success.
I get the following error message "There was a problem uploading your file. "
in options I changed the allowable limit to 15,000,000,000 but nothing works.
You may want to check your normal server logs... it's quite possible whatever is causing it is logging the error with some more info (check web server error logs, PHP error logs, etc.)
There are also settings in your web server that will affect it...
Also, if your site happens to use Nginx, check the client_max_body_size.
I also can't stress enough to not ignore the MySQL settings Jake mentioned. I made the mistake of upping the PHP/web server limit one night, and totally forgot MySQL's max packet size. I tested uploading a big file... it worked and so I left and went to dinner (as in drove away and was gone for a couple hours). Needless to say, the allowed packet size being smaller than the file being uploaded causes all the slave DB servers to halt all replication with the fatal error. That was super fun.
There are three places to change php values: php.ini, config.php and .htaccess.
If possible, changes should always be made in php.ini, but not everyone has access to that, depending on server setup.
Changes made in .htaccess only apply as an Apache module.
Config.php changes will always apply and happen after some XenForo changes.
Certain things however can't be changed in config.php, post_max_size for example, as it must be done before php starts to handle the request.