What’s the format of a Google Docs file? The question may not even be meaningful. According to Jenny Mitcham at the University of York, there is no such thing as a Google Docs file. What you see when you open a document is an assembly of information from a database. You can export it in various file formats, but the exported file isn’t identical to the Google document.
This makes them risky from a preservation standpoint. You can’t save a local backup of a document. If you lose your Google account, or if censorship in your country cuts you off from it, you lose all your documents.
There’s an incentive to turn files stored on Google Drive into Google Docs, because file storage is limited but Docs storage isn’t. This can turn out to be a false economy.
Keeping a local backup of your documents helps. The backup will be in an actual file format, so it may not be exactly the same as what you see online, but it will preserve the content. Jenny Mitcham’s article goes into some detail on how faithfully Docs export into various formats.
My experience with Google Sites has given me concrete evidence of the risks. Some years ago, I set up an internal Google Site for a convention committee I was working with. The site still exists, but at some point over the years it became impossible to edit. The online instructions don’t work.
I don’t know whether this has happened to all Google Sites that are older than a certain age, it was something peculiar to ours, or what. Other people on the committee reported similar frustrations, so it wasn’t specific to my browser setup. Whatever happened, the site was locked into Google, so there’s no way to port it somewhere else without a lot of manual work. It’s plausible that over time, older Google Docs won’t work so well and may even become unusable in some cases.
Proprietary formats are always risky. When the format isn’t a file format at all, you can’t even reverse-engineer it in principle. Google Docs are nice for sharing information in the short term, but relying on them for anything important and durable is a seriously bad idea.
If you’re running up against your allocation limit on Google Drive, you’re using it as an archive, not for working documents. Don’t be a cheapskate. Pay for the storage you need.