Note that PDFTron has other Nuget packages for other products, such as Android and Windows Mobile UWP. This post is just about desktop and server packages.
PDFTron.NET.x64
Linux/macOS/Windows .NET Core 2.1+ 64bit applications
This is the primary package going forward, and will work on Linux/macOS/Windows and will support the current .NET runtimes such as netstandard2.1 and net6.0.
PDFNet
Windows .NET Framework 4.5.1+ 32bit and 64bit applications. If you only need 64bit binary then see package below.
PDFTron.NetFramework.x64
Windows .NET Framework 4.5.1+ 64bit applications.
Since this package only targets 64bit then there is no need for PDFNetLoader.dll to select between 32bit or 64bit PDFNet.dlls are runtime, so this is a smaller/simpler package than the PDFNet one above.