Slim Duiker

Like the original poster, I often used to delete unwanted pages by opening in Mac OS preview, deleting pages and having Evernote update them when I saved them in Preview. This stopped working a while ago - Preview displays a warning on making changes that it is making a copy because it cannot edit the original and when you exit out of it, unsurprisingly the update isn't reflected in Evernote. I've found a new method that works for me though. Right click on the PDF in the Evernote window which offers "Annotate this PDF". In the annotation tool window, down the right hand side, you have thumbnails of each page. Right click on the thumbnail for a page you want to delete and you are offered "Delete page". When you close the annotator, you get a message that the attachment has been updated and sure enough, it has.