The appearance of PDF files within Showcase on Windows machines depends on which version of the Showcase app you have installed.
If you installed via the Windows Store, you have "Showcase ST", and PDFs will open in the default PDF viewer installed on the Surface.
If you installed via a direct download of a .exe file, then you have "Showcase XE", and PDFs will display natively without taking you out of the app.
You can tell the difference between ST & XE by the icons. ST has a fully teal square; XE has a white circle.
If you are using Showcase ST & wish your PDFs would display differently, try Showcase XE! You can download it from https://showcaseworkshop.com/download/.
If you still need to use Showcase ST for some reason, but the appearance/load time of Acrobat or Windows Reader is dissatisfying, consider installing a different PDF reader and making it the default (via Control Panel > Programs > Default Programs > Set Default Programs).
Additionally, if the fact that it's a different app & not native Showcase is dissatisfying, you may find that the experience improves when you use a gesture to switch between apps. On the Surface Pro you can switch between apps relatively quickly by doing a short swipe from the very left edge. This will 'explode out' all currently open apps, and you can tap on the one you want to go back to (e.g. Showcase). Or, if you have the little black toolbar visible down the bottom, you can hit the 'back' arrow to go back to the previous app.