If you have installed using the full MSI installer, .NET Core assemblies are available in the Core3 folder under the ABPpdf menu item. If you have installed using NuGet then it will work out what files you need and put them in your bin directory for you.
However it should be noted that project and assembly management under .NET Core is still somewhat of a work in progress.
Visual Studio does not provide good tools for the management of .NET Core projects which means you often need to edit your project files by hand; error messages are often cryptic and documentation is lacking.
In general NuGet will handle this type of thing for you. However if you run into issues linking ImageGlue into your projects the following may help.
ImageGlue requires references to WPF and System.Drawing for interop reasons. So you need to reference those in your .csproj.
One way to do this is to insert a UseWPF tag and reference the desktop SDK.
You may also find you need to reference System.Configuration.ConfigurationManager.