What browser are you using?
If you're using Firefox go to Tools>Options>Applications
Scroll down to PDF and choose "Acrobat" instead of Firefox PDF reader.
I'm using Chrome, but the problem is present in all browsers. It's actually using the Dropbox Document preview screen, so it's not Adobe or browser-based, in terms of viewing the PDF anyway.
There's nothing you can do about that. When you export a PDF you'll
notice it's an Adobe PDF. All other readers are a crapshoot.
I wish I had better news but I don't.
Yeh - different browsers and applications try to build their own PDF readers and they just don't address all the modules of Adobes PDFs.
You should write a message to Dropbox https://www.google.ie/search?q=dropbox+twitter&oq=dropbox+twitter&aqs=chrome.0.69i57j0l3j6 9i62l2.2474j0&sourceid=chrome&ie=UTF-8
I had a similar issue with an update to Firefox where it wouldn't let me Drag and Drop from the Firefox Library - I wrote to Mozilla on Twitter and it was sorted in a few days.