Hi Shaon,
I installed Download Manager Version 2.6.8 on my WordPress site and I noted the whole site had white screen being rendered on Internet Explorer.
This ‘whole site having white screen’ went away after I deactivated the plugin: Download Manager from WP-Admin > Plugins.
When I activated the plugin, the issue reoccurred.
I noted this phenomena is similar to that in http://www.wpdownloadmanager.com/support/topic/white-screen-after-installation/
Any ideas to remedy this?