Use iTextSharp. It's free and you only need the "itextsharp.dll".
http://sourceforge.net/projects/itextsharp/
Here is a simple function for reading the text out of a PDF.
Public Shared Function GetTextFromPDF(PdfFileName As String) As String
Dim oReader As New iTextSharp.text.pdf.PdfReader(PdfFileName)
Dim sOut = ""
For i = 1 To oReader.NumberOfPages
Dim its As New iTextSharp.text.pdf.parser.SimpleTextExtractionStrategy
sOut &= iTextSharp.text.pdf.parser.PdfTextExtractor.GetTextFromPage(oReader, i, its)
Next
Return sOut
End Function
Now you can search through those files with ease.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…