The article is wrong, the PDF spec has introduced breaking changes plenty of times. It’s done slowly and conservatively though, particularly now that the format is an ISO spec.
The PDF format is versioned, and in the past new versions have introduced things like new types of encryption. It’s quite probable that a v1.7 compliant PDF won’t open on a reader app written when v1.3 was the latest standard.
At a previous employer we did this with our docs repo.
The public docs site was managed and deployed via a private GitHub repository, and we had a public GitHub repo that mirrored it.
The link between them was an action on the private repo that pushed each new man commit to the mirror. Customer PRs on the public mirror would be merged into the private repo, auto synced to the mirror, and GH would mark the public PR as merged when it noticed the PR commits were all on main.
It was a bit of a headache, but worked well enough once stag involved in docs built up some workflow conventions. The driver for the setup was the docs writers want the option to develop pre-release docs discretely, but customer contributions were also valued.
The PDF format is versioned, and in the past new versions have introduced things like new types of encryption. It’s quite probable that a v1.7 compliant PDF won’t open on a reader app written when v1.3 was the latest standard.