I am puzzled as anyone else (maybe). I created a document on the PC desktop, placed (linked) a SVG and saved the document. Nothing unusual so far. After that I created a directory on our server drives, closed the document and moved (or copied) .afpub and .svg there. Then I opened the .afpub on the server drive and now the SVG had the PDF Passthrough option in the context toolbar. I can recreate this at any time with different SVG, no matter saving it on desktop first, then moving it to the server drive or first saving to the server drive and then moving it to the desktop.