How are PDF image alt tags and "subject" field in document properties used for search

[winstoncho]

Hello,

1. Does google use image alt tags? According to this 2011 document, the answer is no, but I have seen others claiming yes- has google since begun using alt tags for images within PDFs?

http://googlewebmastercentral.blogspot.com/2011/09/pdfs-in-google-search-results.html

I am trying to decide if it is worth updating existing PDFs with alt tags for images for the purpose of SEO.

2. How does Google use the "Subject" field in document properties for a PDF? Should it be used as a description field for the document, similar to a meta description?

Thank you!