Short answer:

When searching in the global search box on the dashboard, you must put document numbers containing a space in quotation marks.

Long answer:

CAD Documents can be searched for either in the Dashboard or the dedicated CAD Document search window. In both windows there are different columns where filters can be applied to narrow down a search. For example you can specify a document number as shown below (simply enter the document number, no need for special characters).

On the Dashboard there is the additional option of using the global search box at the top, which will check in the following properties: Definition, Document Number, CERN Drawing Reference and External Reference. However, note that the settings of this search box are different to those of the the filter boxes. In this search box a white space ( ) is seen as an "AND" operator.

Therefore, for components which contain a space in the middle of the document number, in order to retrieve results you must place the 'number' inside double quotation marks (""), as seen in the first image.