TOPIC:

I am currently trying out the free SDK but the library adds a watermark image/text to all my PDFs. Is this only because I am using the free version or does this happen once I pay for the license?

If you're using the demo license, the library adds a blue watermark. If a license is not activated the watermark is red.
Otherwise, when you will buy license, and activate it in the SDK, there won't be no watermark.

For more information about license features:
Test different license types on Android before buying the license