Yes. If someone using Outlook or Notes sends you a meeting invitation that includes an attachment, you'll get the attachment in the email invitation. However, the attachment won't appear in the event on your calendar.
Yes. Click the link to either attach a Google Doc or upload any other type of file from your computer or network (such as a Microsoft Office document, PDF, and so on). Note that you'll have to share any Google Doc you attach with guests for them to view the attachment.