A file with .acroplugin extension is a plugin file used by Adobe Acrobat. It is used to add functionality to the existing capabilities of the application. The plugin comes bundled with Adobe Acrobat, a page layout and PDF file publishing program. Like other plugins, the ACROPLUGIN file extends the exisitng functionalit offered by the application.
ACROPLUGIN File Format - More Information
ACROPLUGIN file is explicitly used by Adobe Acrobat for extending its existing functionality. It’s internal file format details are not available publicly for developer’s reference.