In our application we are placing a PDF attachment in server location.

we need to attach the PDF file to  work object. How to implement this in v7?

Thanks in advance 

There are several ways to attach documents in Pega. Take a look at the online help :

https://pdn.pega.com/sites/pdn.pega.com/files/help_v719/procomhelpmain.htm#concepts/casemanagement/con_conceptsattachments.htm

Am guessing you have some kind of a document mgmt server. Would recommend creating a connection between Pega and that server. And then let users upload documents in Pega and let it move that file into your server. 

Here is an example of connecting to sharepoint : https://pdn.pega.com/community/pega-exchange/sharepoint-2007-ecm