After experimenting with pDF-search for a while, I find it pretty fascinating. The results in PDF-search are much more surprising, than the ones I find in Foxtrot or Devonthink. The reason might be because it uses compilations of algorithms, to rank the pages.

PDF Search algorithm calculates a rank value for every page in documents according to keywords you entered. These ranks are computed as follows:.
Keyword Distance : Pages containing keywords nearer to each other have a higher rank.
Keyword Density : Pages containing more keywords have a higher rank.
Importance : Pages that contain the keywords in the title or that are rendered in bold or a larger font will have a higher rank.
Document Date: Pages within more recent files are ranked more highly than older files.