when I open my fillable form, none of the available fields are fillable.

I am not aware of any OS that does not require a program to read a PDF. Acrobat is a PDF viewer as well as a PDF creator,

After downloading your example PDF and digital certificate, I see the PDF was created using MS Word.  When I used the "Tools => Prepare Form" to examine or create form fields, I see lots of form fields. When looking at the properties of a number of the form fields, I noticed all the examined fields were set to "Read Only", so users are locked form entering data into these fields. It is possible to change them using JavaScript. I also see a number of signature fields and using the Navigation panel it appears a number of these signatures have been applied. Applying a digital signature can lock all form fields.

If you want the fields to be user fillable you must remove the signatures from the form.

You can use the following script to remove the readonly condition from all of the form fields:

var oField

for(var i = 0; i < this.numFields; i++)

{

     oField = this.getField(this.getNthFieldName(i));

     oField.readonly = false;

     console.println(i + ": " + oField.name + " - read only: " + oField.readonly);

} // end field name loop;