I've used pdftohtml to successfully strip tables out of PDF into CSV. It's based on Xpdf, which is a more general purpose tool, that includes pdftotext. I just wrap it as a Process.Start call from C#.
If you're looking for something a little more DIY, there's the iTextSharp library - a port of Java's iText - and PDFBox (yes, it says Java - but they have a .NET version by way of IKVM.NET). Here's some CodeProject articles on using iTextSharp and PDFBox from C#.
And, if you're really a masochist, you could call into Adobe's PDF IFilter with COM interop. The IFilter specs is pretty simple, but I would guess that the interop overhead would be significant.
Edit: After re-reading the question and subsequent answers, it's become clear that the OP is dealing with images in his PDF. In that case, you'll need to extract the images (the PDF libraries above are able to do that fairly easily) and run it through an OCR engine.
I've used MODI interactively before, with decent results. It's COM, so calling it from C# via interop is also doable and pretty simple:
' lifted from http://en.wikipedia.org/wiki/Microsoft_Office_Document_Imaging
Dim inputFile As String = "C:\test\multipage.tif"
Dim strRecText As String = ""
Dim Doc1 As MODI.Document
Doc1 = New MODI.Document
Doc1.Create(inputFile)
Doc1.OCR() ' this will ocr all pages of a multi-page tiff file
Doc1.Save() ' this will save the deskewed reoriented images, and the OCR text, back to the inputFile
For imageCounter As Integer = 0 To (Doc1.Images.Count - 1) ' work your way through each page of results
strRecText &= Doc1.Images(imageCounter).Layout.Text ' this puts the ocr results into a string
Next
File.AppendAllText("C:\test\testmodi.txt", strRecText) ' write the OCR file out to disk
Doc1.Close() ' clean up
Doc1 = Nothing
Others like Tesseract, but I have direct experience with it. I've heard both good and bad things about it, so I imagine it greatly depends on your source quality.
