Best database to store and retrieve Word and Excel documents
Personally since you don't have a relation beyond barcode --> document I don't think a relational database is the best fit.

If your requirements are really as simple as:


Find a document
Serve document to user


Then any Key/Value store should work.  This wikipedia article talks about some of the theory for Document Databases as well as some pro/cons vs Relational Databases.  

You will need to write some sort of interface/application to translate the barcode reader output into a query for the document in the database you choose.  As such you might not even need to use a database at all.  You could just have the application layer find the documents in the network drive...

Since I was pressed for a specific recommendation.  I would look at MongoDB. It should be cheap to run, will fit you needs, and appears to have broad support.  Here is a link to a blog article discussing storing files/binaries in MongoDB.

Fair warning: I don't have any direct experience with MongoDB, and only basic familiarity with document databases in general.  As such my recommendation of MongoDB is based on hearsay, and general impressions.