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Great post. Thanks very much.

No opinions here... just facts.... PDF files show in your Google backlinks PDF files can contain anchor text backlinks PDF files accumulate pagerank PDF files pass pagerank If you place obvious links in PDF files people will click them and land onto your .html pages Other people sometimes grab your PDF files and place them on their own website giving you backlinks from their domain if you were smart enough to embed links within them PDF files can be optimized, rank high in the search engines and pull in a LOT of traffic Some types of content displays and prints much better  in a PDF file than it does on a webpage PDF files allow you to control the "look" of printed documents A huge report is often better posted as a PDF than as html documents You can lock PDF documents to keep others from monkeying with your content (determined people will get around this). Contrary to popular belief, PDF documents can be monetized... just toss in a shopping link or links to pages where money can be made.  I have not heard of anyone paying for ad space in a PDF but there is no reason why that could not be done.

Wait.... your question is so extreme that I must answer in a realistic way... Apple is never, ever, ever going to link to every computer website.  Some of them are not relevant enough to content on Apple's website. Some of them are not good enough for Apple to give them a link. Some computer websites are blackhat. The day that Apple starts linking to everything everywhere is the day that their links are worthless.  Keep that in mind when you start linking out. Keep that in mind when you evaluate the value of a link coming to you. Link only to relevance.  Link only to quality.  Link only to good sites.  If you do that you will have a great site. You keep looking for a "limit".  The limit isn't a number.  The limits are relevance, quality and virtue.