Looks pretty good to me. I have a few suggestions:
1.) Docblocks should not repeat the method name on the first line. First line should be a short summary sentence with no period. Summaries for methods should use 3rd person declarative rather than 2nd person imperative, beginning with a verb phrase. For example, instead of:
* Create a new Scribd account.
* Creates a new Scribd account
2.) Consider using HTTP_Request2 for the HTTP code. HTTP_Request2 already has methods to handle GET/POST argument building and file uploads.
3.) Making the username/password/key static in the main class will restrict you to one Scribd class at a time. If that's intentional, the class should be made a singleton. If not, the properties should not be static.
4.) Use of mb_convert_case() requires the mbstring extension, but it is not specified in the package.xml.
5.) Consider more specific exception classes for different error conditions.