Which app did you start with? Apple Mail, Safari, iBooks or something else?
Is a PDF document already open in other app? If so, you can tap the PDF document to display a toolbar in the app. Do you see the icon like this in the toolbar?
You can tap the icon to open a popup then tap "Open in..." or "Open in Adobe Reader". You may need to scroll to the right to find the red icon with "Open in Adobe Reader".
In Safari, tap "Open in..." (blue text) in the top toolbar, instead.
If you see something else on the screen, please let us know the name of the app.