I have heard of some applicants who got visa but didn't receive pre-arrival services letter, some before their PPR, while others got it just after their PPR. However, anyone that has received the "Medical Request letter" is qualified to use the service.
Since you have gotten that letter, you can register and benefit from the pre-arrival services. If you have not started already, I'll recommend you start using it now to be well prepared to enter & settle in Canada.
Someone has posted the entire letter, but for ease, I am posting excerpts of it that has the links that you need to enjoy thess services.
"If you are outside Canada, we strongly recommend that you use pre-arrival services
before you leave. We encourage you to explore the list of services on the IRCC website:
https://www.canada.ca/en/immigration-refugees-citizenship/services/new-immigrants/
new-life-canada/pre-arrival-services.html
When you register for pre-arrival services with one of the service providers listed on the
website, you will be asked to provide a copy of this letter. You can contact as many
service providers as you like and receive as many services as you need, and these services
are always at no cost to you.
You are also encouraged to take advantage of free Government of Canada resources
available online, such as Welcome to Canada, the official handbook to help permanent
residents prepare to come and settle in Canada. The handbook is available at
https://www.canada.ca/content/dam/ircc/migration/ircc/english/pdf/pub/welcome.pdf.
Canada is a bilingual country where both English and French are spoken. If you are
interested in living in a French-speaking community outside Quebec, please visit
http://www.cic.gc.ca/english/francophone.asp."