PDFBox is the best library I've found for this purpose, it's comprehensive and really quite easy to use if you're just doing basic text extraction. Examples can be found here.
It explains it on the page, but one thing to watch out for is that the start and end indexes when using setStartPage() and setEndPage() are both inclusive. I skipped over that explanation first time round and then it took me a while to realise why I was getting more than one page back with each call!
Itext is another alternative that also works with C#, though I've personally never used it. It's more low level than PDFBox, so less suited to the job if all you need is basic text extraction.
with Apache PDFBox it goes like this:
PDDocument document = PDDocument.load(new File("test.pdf"));
if (!document.isEncrypted()) {
PDFTextStripper stripper = new PDFTextStripper();
String text = stripper.getText(document);
System.out.println("Text:" + text);
}
document.close();
PDFBox contains tools for text extraction.
iText has more low-level support for text manipulation, but you'd have to write a considerable amount of code to get text extraction.
iText in Action contains a good overview of the limitations of text extraction from PDF, regardless of the library used (Section 18.2: Extracting and editing text), and a convincing explanation why the library does not have text extraction support. In short, it's relatively easy to write a code that will handle simple cases, but it's basically impossible to extract text from PDF in general.
Source: Stackoverflow.com