The PDF file itself does not control the "print as image" preference, which should always be turned OFF unless there's a specific need to rasterize the entire page, and is turned off by default in all Acrobat and Adobe Reader installations.
What's the filetype for the ultralight font and what export setting* are you using in ID? If you're exporting to earlier than PDF1.7, OpenType fonts will be converted and may experience some small changes in shape if their hint definitions aren't done right.
*As a side note, bear in mind that InDesign does not use any part of Acrobat when exporting PDFs, so questions specific to that stage in the process should be posted to the InDesign forums.
Hey Dave thanks for your reply.
I don't think it is an issue with the export setting or any part of ID (I had originally posted there), since the same exported PDF prints fine in preview. It is only in acrobat that the fonts seem extremely light. I am using a TT font, and have exported the PDF in High Quality Print and Press Quality, both of which have had the same issue.