Q: PDF's in Safari not displaying

Safari - Unsupported third-party add-ons may cause Safari to unexpectedly quit or have performance issues

Safari doesn't load a page or webpage items are missing

Safari/other browsers – Website not loading

Maybe you can upgrade your system to Sierra now, also update your Safari to the latest. I am using Google Chrome and it works good to open PDFs, there's a lot features you can make use of with Google Docs.

Adobe Acrobat X 10.* products reached their 5-year, end-of-life cut-off in November 2015. Due to this Adobe decision, there will be no further updates of this version family for compatibility with later releases of El Capitan (e.g. 10.11.6), macOS Sierra (10.12), or (inherently) Safari 10. See Product End of Life schedule at the end of this linked Adobe Devnet page.

Only the current Adobe Acrobat Pro (and Reader) DC products show specific compatibility with El Capitan and macOS Sierra. Still, that does not explicitly assure thorough compatibility with Safari 10. Adobe is more detailed about their browser support for Windows than Mac.

If you click on a link expecting an embedded PDF in Safari 10, and you get a gray embed instead, then the PDF document itself is missing (or moved) on the server. Adobe has a PDF entitled, “Parameters for Opening PDF Files” (direct link to PDF) that governs how one controls an embedded PDF view in a browser.

Here is this document opened in Safari 10 on macOS Sierra (works on El Capitan too) — using the embedded syntax discussed in that PDF. Not all controls are used, and the PDF embed is using the center of the browser view. Note: This was done on a MacBook Air, and not all of the embedded effect is shown due to screen real estate.