Do search engines crawl PDFs and if so are there any rules to follow when making them

Google definitely indexes PDF files and you can search just for PDF files by adding filetype:pdf to your search query (example).

I would say the main things to do to optimise a PDF so it's easily indexed would be:

• Give it a meaningful filename
• Complete all the document metadata properties (title, author, keywords etc)
• Make sure your PDF is comprised of actual text and not scanned images
• Ensure you have good content with correct use of headings, just as you would an HTML document

For more tips read Optimizing PDF Documents and Eleven Tips For Optimizing PDFs For Search Engines