Google’s John Mueller said that Google will convert PDF documents, as well as other documents, from their original form and into HTML in order to better index those documents.
John said on Twitter “FWIW we convert PDFs & other similar document types into HTML for indexing too.” We also know that Google is pretty slow when it comes to reindexing PDF documents – so keep that in mine when updating those documents.
I assume other documents that Google converts from their original form to HTML for indexing is not just PDFs, but also Word Documents, Excel, some images, and other documents that may contain text.
The trick is to format the PDF with markup, including Schema, for maximum conversion value and algorithm understanding.
Most create PDFs without understanding SEO value needs.
Forum discussion at Twitter.