New scoring function for PDF files

The scoring function for PDF files has been changed to achieve two goals:

1. By default, PDF files are penalized so that they appear lower in search results than other document types, such as HTML. This has always been the case, but the penalty has been increased. Still, highly relevant PDFs may appear before less relevant HTML.
2. The /pdf0 option now imposes a much greater penalty. Appending "/pdf0" to a search URL adds a significant penalty to PDFs. In general this means that in the search results PDFs will follow other file types, such as HTML. However, as in the default case, very relevant PDFs may rise in the search output.