To use fillable PDFs, your students will need PDF readers on their computers or devices. The PDF reader must be able to detect form fields (those are the places where students type their answers). Adobe Reader is a free download or app available for most computers and devices. If you've got Chromebooks, try having students fill out the PDFs using pdfescape.com.
You should be able to distribute the fillable PDF files to your students using the digital platform you’re already using. Examples include Edmodo, Canvas, Google Classroom, or Apple Classroom. Again, it won’t work unless students have PDF readers.