It depends on how the PDF was created. If it was generated from a text file, you should be able to just use the Text tool in Preview, highlighting the text you want and copying/pasting it.
This doesn't always work though. Some PDFs are copy-protected, so you'll have difficulty copying text out of them unless you have the password to unlocked them. Also, a PDF could simply be a scanned document; if no optical character recognition software (OCR) was used in the scanning process, you won't be able to copy text out of it without running it through good OCR software yourself first.