open pdf files

There are two possible answers to your question.

First, you can download and use the new Acrobat DC plugin (AEM Mobile PDF Export for Acrobat) to export the PDF document from Acrobat into AEM Mobile (as a .article file). Then just configure an image card in the layout with a navto:// link to point to the new converted PDF article. You can learn more about the Acrobat plugin here, Adobe Prerelease.

The second method is more involved and requires programming, but it gives you more flexibility in how you do things. When a content view (an .article document) loads, you can include Javascript in that view that fetches a PDF document from an external service and stores it in the local filesystem (using the new Cordova File System and File Transfer APIs). Once stored, another link on that page could fetch the PDF document from local storage and open it in the default handler for PDF content on the device. You could also create a first-launch content view that fetches multiple PDF documents and saves them so they're available for offline use in the future.