Addition to the to the approved answer: there are also alternative commercial solutions to replace Adobe IFilter for text indexing (providing the similar API but also offering additional premium functionality):
- Foxit PDF IFilter: provides much faster text indexing comparing to Adobe's plugin.
- PDFLib PDF iFilter: includes support for damaged PDF documents plus the additional API to run your own queries.
If you are looking for the single tool that can be used from both managed .NET apps and legacy programming languages like classic ASP or VB6 then this is where the commercial ByteScout PDF Extractor SDK would fit as it provides both .NET and ActiveX/COM API.
Disclaimer: I work for ByteScout
