The trial version allows most features to be used. You are permitted to save images 35 times before full demo mode is activated and you won't be able to save anymore.
For the best experience, your PC needs to have 1080p or higher screen in 100% screen scaling, and the .NET Framework 4 installed for a small number of plug-in features. Your computer's processor (Intel, AMD) should be recent enough that it can run SSE2 and AVX extensions. Shader level 6 is required for GPU functions.