You can disable file syncing in the Sync pane of the Zotero preferences.

Previously synced PDFs have not been removed. You can use Purge Storage function to do so. See https://forums.zotero.org/discussion/28958/purge-storage/ for description and https://www.zotero.org/settings/storage for the actual function (you will probably also want to reset _file_ sync history in the Zotero client afterwards too, so you don't forget to do it in the future. BE CAREFUL WHAT YOU RESET!)

Beforehand, could you tell us why you want to delete uploaded items? If you stopped file sync altogether, there is generally no need to worry about storage space even if it's full (though I can think of a few scenarios)

Also,

Since I would like to continue syncing my records (without attachments) to the free online Zotero storage, I am clearing already uploaded PDFs to clear space for just my records. Maybe records take very little space and I do not need to clear PDFs for that. But if not, I don't want to miss updating a record because of no space. Does this make sense?

When I first started using Zotero last year, I would create an entry (mostly scholarly articles and reports) and attach a snapshot of the page and the PDF. I have now stopped attaching PDFs because I want only one copy of the PDF on my drive and I want that copy to be the 'original' copy I saved, read, highlight, and comment.

You can still use attached PDFs. When you attach a PDF, it copies it to the zotero storage directory, so you would just delete the PDF you attached (e.g. on the Desktop). Accessing the PDF without Zotero's help becomes difficult in this case though. Either way, what you're doing with file links is perfectly fine and will probably work out better in your case.

I'd like to remove PDFs from the online storage just to stay consistent - one and only one copy of the PDF, because I highlight and annotate my PDFs and want to see those notes.

In this case, whether you purge online storage or not will make no difference. If you ever do decide to start using online storage again, all files that you deleted locally will also be deleted online when you sync at that time. If you don't decide to start using it, then you will never even notice that those files are taking up storage space.

I would purge anyway and then reset file sync history like I say above. One scenario that I was thinking of is if, say, you wanted to not sync attachments in your personal library, but still wanted to sync file attachments in a group library. Then purging storage and clearing out attachments from your personal library online would clear up space for the group library.

Maybe there is a better way, probably using Dropbox. I am not aware of the steps and until then, would like to avoid multiple PDFs, only one or some of which show my notes.

Since you are using linked attachments, you can easily use Dropbox to sync them across computers. See "Relative Linked Attachment Filepaths" here. I'm not sure how you have organized your PDFs right now, but, say, you have them all in a single directory. What you would do, is set the Base Directory to the current directory where they are stored. You would then move that directory to Dropbox. Go back to Zotero and now change the Base Directory to that same directory in Dropbox (hope that makes sense). Your files will now be on Dropbox and accessible from Zotero. You will then sync Dropbox to other computers and set their Base Directory to the attachment directory in Dropbox.

You may also find ZotFile useful. It can help you convert all of your attached PDFs into linked files, move them to a specified directory, and rename the files based on the metadata in Zotero.