Nothing, except Word, is 100% compatible with Word.
Even that doesn't always hold true. Different versions of Word will not always interchange documents well. For example, older versions of Word will not always process documents from newer versions of Word.
Before I retired, this was quite an issue with so many people writing stuff in Word so the company standardized on one version of word and installed it on all systems.
Pages can open word documents, but it won't be perfect. The more complicated the document the less likely it will read properly in Pages. Interestingly enough, even simple documents do not always work; Word includes a number of fonts that are not available in Pages, so documents can look strange when opened in Pages.