What is the blue symbol in the top right corner of my iPad?

The icon indicates that your iPad is synchronising with "something". Your screenshot also indicates that a download is in progress - the progress bar suggesting that the download is ~30% complete.

As for what the "something" is, you'll hopefully have some idea as to what your iPad is connect to, or the App(s) that you are running on the device. Is the iPad physically connected to a dock or external USB storage device?

Speaking of personal experience, the download and synchronising icons are fully independent of each other. I too am running iPadOS 18.2 on an iPad Pro.

I have just downloaded a ~1GB file from a website - and see only the download icon. The other icon of which you enquire does not relate directly to the download process.

There is no reason to be concerned. Yours is at least the third report that I have seen in recent days.

Apple's documentation is often slow to catch-up with new features. Here is another status icon - that appears briefly when connecting some USB-C hubs - that Apple have yet to add to their documentation:

You are very welcome.

Hopefully I will soon be able to provide a definitive answer. While I heavily use my iPad Pro on a daily basis (this being my computer of choice for the majority of tasks), I am yet to see the status icon that you and others have seen.