Thanks for reaching out nitinK76,
It sounds like you're having trouble using the web version of the Outlook app in your browser. We can help.
A good place to start is by checking to see if you can access the PDF using either a different device, different browser, or different network. This will help you to isolate the cause of the behavior.
If the issue is with the device, some steps you can start with include restarting and trying again. Restarting the device can help with intermittent issues. If the issue is with the network, it could mean that there is a security setting that is blocking access to the site. If you find that the issue only happens in a particular browser, it could mean there is an issue with an extension, or that you might need to clear browsing data or cookies.
Additionally, Microsoft makes this support page available as well: Cannot Preview PDF attachments in Outlook Web Access.
Hope that helps.
Take care.