Q:

Documents in Customer Communties

Am trying to add help link in a visualforce page which points to pdf uploaded in documents tab.But looks like documents is not available in Communities.What's an alternative for adding this help document in communities.

A:

Here's some advice. Take it or leave it.
Exposing documents externally is possible in salesforce, but it's not easy or scalable.  For our communities we have switched over to an Amazon S3 bucket. 
The advantages:

The process of uploading and exposing documents is easier in AWS, which in turn makes it easier to train people. 
URLs were easier to CNAME. Marketing loves those pretty URLs.
More space! AWS provides an easier path to upgrade storage.

If you want to use salesforce you can use Chatter Files. see this post for more information : Access Document(pdf or doc) without Salesforce login

