John Mueller from Google stated that Google will be converting the PDFs and other documents into HTML from their actual form for ensuring a better indexing of the documents.
John posted on Twitter saying that for what it’s worth, Google converts PDFs and the other same kind of documents into HTML for the purpose of indexing. Google is really slow when re-indexing PDF documents are concerned. So, you need to keep that in mind while you start to update the documents.
The other documents converted from their actual form into HTML for indexing are not simply PDFs, but excel sheets, word documents, images, and documents containing texts. An SEO consultant named Alan Bleiweiss also mentioned that utilizing schema and another such markup will also help Google in understanding the PDFs better.
See the forum discussion at Twitter.