We are using the Bamboo Data-View Web Part in SharePoint 2013 to allow users to filter and export results from a single Library and I have just discovered that the Filter is hidden in a column when you have a Multiple Lines of Text in the grid. This makes the tool pretty useless now so I was hoping that someone has come across a better Web Part?
So the answer is that Multi-line Text fields in SharePoint are NOT searchable or Filterable, end of story. I have found a solution that works. This solution is to keep a SQL Server table update to date with the library data, less the documents of course, and use a nvarchar(4000) column for the multi-line text fields. We will never exceed that number of characters so this works great plus it's much faster than rendering a SharePoint Library!