Will everything be gone?

I have been using Microsoft Word on my laptop for a long time, it was pre-installed and I did not have to log in. I have created many important documents there for me. If I decide to register there, will all the documents be gone? I would like to register to use the app on my mobile phone.

Your documents, if they are all stored on your PC or somewhere else, are guaranteed not to disappear.
You should always make a backup to protect your data from mishaps.