Not sure what could be wrong, however here are some of the steps I would take and things I would check.  

I would check a couple of settings.  Check the user permissions in the global settings to ensure user is able to upload files is checked.  Next I would check the plug in that you are using that allows the upload of the file (if you are using one) and see if there is anything in the plug in that needs to be activated to allow upload to another user's wall.  Also does the plug in actually allow a file to be posed to another users wall or just the user's own wall?  Finally I would check the file size of the PDF and ensure it is not larger that the allowable maximum file size.  I realize it does not make sense to get an extension not allowed error message when the file is too big.  But then stranger things have happened.  I wish you will in you quest to solve your issue.

@Sandy Hill Thanks for the reply.

If you try to upload a file using demo.oxwall.org, you can't upload to it. It is a feature which is not working in oxwall's core functionality.

Guna, concerning the file type formats allowed for uploading, this question has already been discussed: http://www.oxwall.org/forum/topic/32510?page=1#post-134852 .
In the meantime to change the uploading file type format for the Wall widget and Newsfeed plugin is considered as a custom code modification. I'm not sure if there some current plugins with such functionality, but you can try searching one in our Store. May be this plugin will be useful for you: http://www.oxwall.org/store/item/425 . But you need contact the plugin developer and ask him whether this plugin will be compatible with 1.7.2 version of the Oxwall software.

Thanks Taissa for your reply. I like to know that Does Oxwall allow to upload PDFs/docs through wall post normally?

Topic was moved from General Questions.

@Taissa I like to know that Does Oxwall allow to upload PDFs/docs through wall post normally? Does anybody know about the files allowed in newsfeed?