I am working on a lengthy report in InDesign. There is a large table that I would like to make scrollable (as in Excel with a fixed header row, etc). Is this possible? Saving to an epub is not ideal, I need to save as a PDF.
There is no way to do that with InDesign. You can check with the experts over in the Acrobat forum but keep in mind that anything like this would require some scripting that would probably not work on any other PDF readers.
What you're asking isn't really possible in PDF using native features.
If your final product will be deployed to a limited group within which the computing platform is known and uniform (i.e., everyone who'll get it is in your organization, and you know they all run Adobe Reader and Excel), you could include the spreadsheet as a PDF file attachment, and it will open in Excel on the recipient's computer.
If it is to be broadcast to many users "everywhere," the better bet would be to post the spreadsheet as a table to a universal online resource like Google Docs and link to it in the PDF.