Victoria,
You can simply create a new note and then drag and drop the Pages document into the Note.
When you double-click on that embedded Pages document, it will open in Pages.
Notes imports are restricted to text-oriented documents as discussed in Import and export notes on Mac - Apple Support. For instance, if I took that original Pages document and exported it to RTF (or RTFD in this case), then the Notes import chooser would reveal the RTFD and import the equation in that document:
RTFD because the equation in that Pages document is an embedded MathML formula.