It is correct to say that a PR must meet the PR RO each five year period. But I get the impression there is some confusion about what "each five year period" means.
As of the fifth year anniversary of the date the PR became a Permanent Resident, "each five year period" is determined daily, each and every day, and that period is the five years preceding that day.
That is, there is in effect a new five year period each and every day, and to be in compliance with the PR Residency Obligation the PR must have been in Canada at least 730 days for each and every day during the five year period preceding that day.
Returning to the example of the PR who landed and became a PR on August 21, 2015, to be in compliance with the PR RO that PR must:
-- be in Canada 730 days between September 1, 2015 and September 1, 2020
-- be in Canada 730 days between November 17, 2015 and November 17, 2020
-- be in Canada 730 days between January 21, 2016 and January 21, 2021
-- be in Canada 730 days between July 23, 2016 and July 23, 2021
-- be in Canada 730 days between July 24, 2016 and July 24, 2021
-- be in Canada 730 days between December 1, 2017 and December 1, 2022
And so on . . . That is, pick a date after August 21, 2020, any date, and as of that date if the PR has not been in Canada 730+ days within the five years immediately preceding that date, the PR is in breach of the PR RO and subject to being deemed inadmissible, subject to losing PR status.
Moreover, I concur in Buletruck's observation about it being risky to be abroad for such extended periods of time that the PR is cutting-it-close relative to compliance with the PR RO, that PR status is about settling permanently in Canada with generous allowances for the contingencies in life compelling PRs to sometimes spend longer periods of time abroad (up to a maximum of 1094 days over the course of any five year period of time).
In an effort to make this as clear as possible: it appears that you have an impression the clock starts over after the first five years. As noted above, that is NOT how it works. Again, the expiration date on the PR card is NOT relevant . . . the only thing the expiration date on a PR card is relevant for is that the expiration date is the last day that card can be used to board a flight coming to Canada from abroad. Has nothing at all to do with determining if the PR is admissible, if the PR is in compliance with the PR Residency Obligation.