You can definitely save as .png to your device, you're just not quite looking in the right place to do it. Share to Behance is in the first Share menu (in Project view), you need to get to the second Share menu (which is in Edit mode).
So, open your drawing like you're going to work on it. All of your drawing tools and layers should be visible and you should be able to draw. Then tap on the Share icon in THAT upper navigation and you'll see a Share option. Tap Image and that will open a menu with a number of different options for sharing/saving the image.
Let me know if you have trouble.
Thank you! You are correct that I was not in the correct menu but had gone too far. Glad to see that works.
Good. Glad you've got it figured out.