I want to add a signature in each page of pdf so my question is can we use the third party DLL like itextSharp in provider hosted app in SharePoint?
2 Answers
The benefits of a provider-hosted App (if any...) is to off-load all the server-side code to a "third-party" server. This third-party server is anything you like (even PHP or Java if you like to rebuild all the oAuth token stuff in these languages). It is hosted anywhere you want (on your own premises, in the cloud, in Azure, ...). SharePoint/Office 365 {don't/cannot/doesn't want to} control what happens there.
You can do whatever you want with it (as per the policy of the hosting service you chose), including loading libraries, other DLL, etc.
So, in a word: yes, you can.