What happens when you try to open it? If the "Print/Save Statement (PDF)" link on the My Account billing page doesn't work, try the "View Bill History" link on the main Billing page to reach https://customer.xfinity.com/#/billing/past-statements. There, click the "View PDF" link for the current bill. That usually works for me, but sometimes I just have to wait a day or two until Billing gets its act together and fixes the links.
Wanting to look at my last bill un able to pull it up.
Are you referring to your most recent bill, or your final bill after canceling? Are you getting the "account restricted" notice?