Safe Boot mode is not the first priority in restoring Pages to nornal operation, though it is good that your Pages v6.3.1 is now fixed.
Ordinarily, unless the Font database is corrupted, or fonts critical to Pages normal operation have been deactivated, or removed — simply holding down the shift key while launching Pages, followed by opening the document — is frequently sufficient to cure an initial Pages crashing issue.
When that does not work, then proceeding to Safe Boot mode has also been shown over time to resolve many other unexplained Pages behavioral issues, perhaps partly in the regeneration of a System Font database, clearing of System caches, or not loading certain conflicting software drivers.
I do not run any anti-virus, or so-called Mac cleaner products on my Macs, and they continue to work flawlessly with routine oversight. Over time, I have seen a great many posts in the support communities about cleaner products aggressively removing parts of the operating system, or installed applications — that lead to unstable systems, or bewilderment at erratic or failed application behavior. I do not recommend these products because they can cause difficult to diagnose problems.