I have noticed that PDF files seem to do well in search results even if the document is a number of years old. Is there any particular reason behind this?

Just curious

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It doesn't have to be number of years old Noiseair. Generally search engines such as Google goes index PDF file. So if it is an ebook, I would password protect it or block the search engine from accessing the page with the use of META robotx tag or robots.txt exclusion protocol.

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I personally block search engines from PDFs.

If someone clicks through from the SERPS to a PDF, they are usually on a dead end page. Having read the PDF, where do they go? If the PDF has a link within it to the site it came from then humans (and search engines) can follow that link, but that is a very poor substitute for the website menu and banner with all the other stuff you want a visitor to see and get them clicking around your site.

If you're offering a PDF as a download, then I'd make it non-spiderable and have a spiderable HTML version on your site. To my mind, PDFS and Word documents are for taking away from the web with you.

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