Thanks for asking and for looking into it.
Here's the problem (with Evernote premium on iPad Pro w/iPad OS14.4 and latest EN update installed, I attached a sample):
1. I open the note (as advised by you), then
2. as advised I click on edit and can edit and annotate the note BUT not the PDF within it,
3. then I click on the PDF-file in the note. The PDF opens and fills the whole screen, but
4. there's no edit option, neither are there circles, pencils or an annotation mode for the PDF itself
Most grateful for any hints,
best, D