The API 688 PDF can be purchased legally through official standards distributors such as: The American National Standards Institute (ANSI) Webstore. IHS Markit / S&P Global Standards Store. Techstreet. Implementing API 688 in Projects
If you are looking to download the official PDF for a project, keep the following in mind: Review of API 618 & 688 Editions | PDF - Scribd api 688 pdf