Hello Vincent,

As a best practice Salesforce recommends to use "Salesforce CRM Content" to store all documents like trials, newsletters and white papers and related information

With Salesforce CRM Content you can organize, share, search, and manage content within your organization and across key areas of the Salesforce application. Content can include all file types, from traditional business documents such as Microsoft PowerPoint presentations to audio files, video files, Web pages, and Google docs.

Salesforce CRM Content simplifies content management by incorporating user-friendly features into the following tasks:

Organizing
Searching
Subscribing
Previewing
Contributing
Reviewing Usage and Providing Feedback
Sharing Content in Salesforce

To set up Salesforce CRM Content for your organization:

Enable Salesforce CRM Content from Setup by clicking Customize | Salesforce CRM Content | Settings and selecting Enable Salesforce CRM Content.

Then you can relate this to Lead, Contacts and Account by adding related list "Content Deliveries" from respective page layouts.

For complete details you can download and refer Salesforce CRM Content Implementation Guide from following link :

https://ap1.salesforce.com/help/doc/en/salesforce_content_implementation_guide.pdf

Regarding integrating with sharepoint, dropbox and sync app, you can find apps in appexchange: Forexample for sharepoint we have:

https://appexchange.salesforce.com/listingDetail?listingId=a0N30000001rvrJEAQ

https://appexchange.salesforce.com/listingDetail?listingId=a0N300000055X6zEAE

If you do not find any related then you can integrate with the help of webservices, either SOAP or REST based.