You will be required to submit the PR - Immigration medical examination (IME) document (will be provided by the Panel Physician doctor post your medical exam) as part of e-APR checklist documents, while submitting your PR application to IRCC.
Most of the Panel Physicians ask the applicants, if they have an invitation before performing a IME. Since you already have an invitation, you are good to proceed and finish your medical exam. Meanwhile, you can work on gathering the remaining documents based on your personal checklist information in your PR application (you can view the checklist online, once you fill in the PR application details).