Bernardo,
In the context of your scenario the users are identified by the built-in visitor IDs rather than by a custom dimension you may have set with a custom unique identifier (unless you set it in s.visitorID variable).
Create a segment with a Visit container and add a condition that filters PDFs and the number of their downloads. In the out of the box implementation that could look as download link instances equals 3 AND download link ends with .pdf.