Google first started indexing PDF files in 2001 and currently has hundreds of millions of PDF files indexed. Today, the Google Webmaster Central blog published a collection of the most often-asked questions about PDF indexing.
Here are the Q&A's from the Webmaster Central blog post:
Q: Can Google index any type of PDF file?
A: Generally we can index textual content (written in any language) from PDF files that use various kinds of character encodings, provided they're not password protected or encrypted. If the text is embedded as images, we may process the images with OCR algorithms to extract the text. The general rule of the thumb is that if you can copy and paste the text from a PDF document into a standard text document, we should be able to index that text.
Q: What happens with the images in PDF files?
A: Currently the images are not indexed. In order for us to index your images, you should create HTML pages for them. To increase the likelihood of us returning your images in our search results, please read the tips in our Help Center.
Q: How are links treated in PDF documents?
A: Generally links in PDF files are treated similarly to links in HTML: they can pass PageRank and other indexing signals, and we may follow them after we have crawled the PDF file. It's currently not possible to nofollow links within a PDF document.
Q: How can I prevent my PDF files from appearing in search results; or if they already do, how can I remove them?
A: The simplest way to prevent PDF documents from appearing in search results is to add an X-Robots-Tag: noindex in the HTTP header used to serve the file. If they're already indexed, they'll drop out over time if you use the X-Robot-Tag with the noindex directive. For faster removals, you can use the URL removal tool in Google Webmaster Tools.
Q: Can PDF files rank highly in the search results?
A: Sure! They'll generally rank similarly to other webpages.
Q: Is it considered duplicate content if I have a copy of my pages in both HTML and PDF?
A: Whenever possible, we recommend serving a single copy of your content. If this isn't possible, make sure you indicate your preferred version by, for example, including the preferred URL in your Sitemap or by specifying the canonical version in the HTML or in the HTTP headers of the PDF resource. For more tips, read our Help Center article about canonicalization.
Q: How can I influence the title shown in search results for my PDF document?
A: We use two main elements to determine the title shown: the title metadata within the file, and the anchor text of links pointing to the PDF file. To give our algorithms a strong signal about the proper title to use, we recommend updating both.
If you want to learn more, watch Matt Cutt's video about "What are the best practices for PDF optimization?" video blow, and visit Help Center for information about the content types we're able to index, Google explained.