And what if those "cloud based office tools", which would be using proprietary file formats internally, shut down? Or heck, what if they announce to raise their pricing? All of your docs gone. No possibility to migrate.
When the original application is open source, its format is public and anyone who wants to create a converter to another format can either use the original code or inspect it to see how the format works.
When the format is proprietary, you get a blob of binary data that you have to reverse engineer through guesswork, made orders of magnitude more difficult if the original service isn't even available anymore.
PDF is a mess because it's an ancient originally-proprietary format based on a proprietary language and the Adobe "standard" didn't actually specify a lot of important things for a long time, leaving other implementations to try to reverse engineer them.
Meanwhile you look at actual standard formats like PNG or UTF-8 or, as you say, HTML, and the implementations generally do a lot better because there is a proper spec.
And they don't share a universal format.