There are no languages that allow you to load a PDF, because that's not what languages do.
You need a library you can use in your application that can load the PDF and allow filling in boxes, like GdPicture.NET
] or GrapeCity's library
Which library you pick doesn't depend on the language so much as the application type, like Windows Forms, ASP.NET, WPF, ...
Google for "C# pdf library" for others, but if you need to fill in forms, don't expect the library to be free.