login Authentication mechanism for Dam assest in publish instance

1. If you have multiple PDFs/assets which require authentication then why not create an assets listing page which can be shown to authenticated users. You can set CUG (Closed user group) properties on the listing page so that only required group of users will have access to it via login. 
2. If you want to restrict access to a particular PDF or asset then there is a way to set the CUG properties on individual assets. http://www.wemblog.com/2013/01/how-to-associate-cug-with-dam-asset-in.html
3. If you do not like applying these properties individually on multiple assets then other solution could be to create a custom servlet and bind it to a resource type and bind that resource type to a page. Then you can restrict access to that page by setting CUG properties on it. You can then expose the page url with asset path as parameter in the front end to the users. When they hit the page URL the AEM will check if the user is authenticated or not. If not then it will redirect to the login page and if yes then it will call the servlet and pass the asset path as param to it. The servlet then can redirect to the asset URL(passed as param to it). 

CUG - https://docs.adobe.com/docs/en/aem/6-1/administer/security/cug.html

https://helpx.adobe.com/experience-manager/using/resourcetypes.html

https://cqdump.wordpress.com/2015/03/23/aem-coding-best-practice-servlets/

Thanks,

Kunal