Hi Ram ,
This looks like your client machine specific issue.
You dont need adny extra add-in to view PDF file. ALM explorer just invokes Adobe reader.
1 ) Is this happening on many ALM client machines ?
2 ) Have you tried to open PDF attachment from IE browser on the same machine ? Does the issue happen there as well ?
3 ) Does this happen to all PDF files or few ?
4 ) Does this happen accross multiple projects ?
5 ) WHat is the size of the PDF file ?
6 ) What is the build number of ALM explorer ? In the ALM explorer , yopu can click on "About" to know the build number.
Regards,