Google's PDF viewer is now integrated with Google's search results, replacing the "view as HTML" option. Google converts PDF files into PNG images, but the files are still searchable and you can copy some of their content.
Not all the PDF files from Google's search results include the new option, so it's likely that Google doesn't perform the conversion on the fly and not all the files have been converted.
The PDF viewer is also used for the PDF files uploaded to Google Docs and for Gmail attachments.
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