Update by David M. Longo, Ph.D.
On August 25, 2022, WIPO released an updated version of its helpful “Introduction to the International Phase” and “Introduction to the National Phase” documents, which are available at:
Introduction to the International Phase: https://www.wipo.int/export/sites/www/pct/guide/en/gdvol1/pdf/gdvol1.pdf
Introduction to the National Phase: https://www.wipo.int/export/sites/www/pct/guide/en/gdvol2/pdf/gdvol2.pdf
Of course, country-specific annexes to the International Phase and National Phase guides can also be obtained from WIPO here: https://www.wipo.int/pct/en/guide.
As an aid to users, WIPO provides the most recently updated date at the bottom right corner of each page of a given guide.