I don't understand why, if you want to publish a PDF to a web site, you would create a portfolio. What you have now is a PDF that can't be viewed by the great majority of web users. What led you in that direction?
I have a 2157 page PDF that I do not want to post to the web for users to view/download. I want to Publish it. If you have InDesign, this is the same feature as Publishing an InDesign document. This feature publishes an Adobe flash file that allows the user to page through the document and view on screen but not download.
Here is the Adobe user guide for the feature: Publish and share PDF Portfolios
Here is an example: https://indd.adobe.com/view/deaa558b-364a-42a1-a1af-98dcd3ce2f9e
Ok, I understand what you want to do. Never heard of that feature before, so I'm sorry I don't know why it isn't working for you. In the medium to short term you might consider that Flash has a death date of 2020, and will in practice likely be turned off much sooner.
After experimenting, I found that if a PDF is one of the documents in the Portfolio then the option is grayed out.