Agent Ransack matches regex on a line-by-line basis, ie each line is matched independently of the others. Multi-line regex is only available in FileLocator Pro.
I think the best option is to search for '*' as a Boolean expression and sort on the 'Hits' column to group files with the lowest number of hits, e.g.
File name: *.pdf
Containing Text: *
Look In: C:\Search
However, one issue you will have is that the PDF Reader adds in ' Page 1 ' type markers which will count as matches. You can mitigate against this with the Pro version by switching to using the installed IFilter instead (Configuration->Extensions->PDF Settings):
You should get something like this: