a!documentviewerfield not rendering PDF type files when clicked.

This looks like an unusual issue. Are you getting any error in the std-out.log file?

How will that be helpful in this situation ?

We only know this aftter reading the file. Could be some issue with the user's context. Or something that Appian isn't able to show on the front end. 

Also, here are some point you might want to confirm. 

1. Dev team is trying to access the file on the same environment as the end users. 
2. The document ID you are trying to pass to the viewer is a valid document ID
3. The PDF is a valid PDF and you have access to it, plus you are able to open it if you download. 

Thank you for the answer will look into the std-out.log file for more insight, please find my answer in red

1. Dev team is trying to access the file on the same environment as the end users.- Yes tried it on client environment
2. The document ID you are trying to pass to the viewer is a valid document ID - Yes
3. The PDF is a valid PDF and you have access to it, plus you are able to open it if you download.- Once downloaded the PDF is opening as expected

I assume that you return the document id from that process.

Did you enable chaining?

What is the runtime of this process?

Do you see the returned valid document id in your local variable?

Added to the previous suggestions - did you double check the security settings the file gets downloaded under (which is probably just the security for the folder it's loaded into)?  If admin users have no issues while some other users are failing to see it, in my experience the first thing to check has always been whether or not those users even belong to the necessary groups to view the folder contents.

Yes we did Check for the permission for the folder the user have required permission and the Document is also getting downloaded every time into Appian folder after clicking on the provided link.

Yes we are getting valid document ID and the attachment is getting stored into Appian folder as well  temporarily.

To double check, add a Document Download Link on the same form pointing to the same PDF file, and for the users who can't access it via the Preview field, see if they can still download it.

Also, can you reproduce this behavior logged in as different users on the same machine?  If the problematic users are all at different locations, it could have to do with local firewalls blocking the special Appian URL used for hosting preview document material, which if i remember correctly, is something slightly different.