Having trouble using AirDrop between iMac and iPad

I'm trying to use AirDrop to send a copy of one of my Numbers spreadsheets from my iMac to my iPad. My iPad Air 2 running iOS version 12.4.1 has the Numbers app on it. I have my spreadsheet, "Mag. Subscriptions on Desktop" — it exists on my Desktop — open on my iMac (27 inch, mid-2011, macOS High Sierra version 10.13.6). On the iMac, with my spreadsheet open in Numbers, I try to use the Numbers Share menu to select Send a Copy > AirDrop. That gives me a drop-down box titled "Send via AirDrop" and allowing me to select among Numbers, PDF, Excel, CSV, and TSV versions. Numbers is automatically highlighted in that drop-down. (The drop-down also has a "Require password to open" checkbox, but I leave that unchecked.)

When I click on the Next button on the drop-down, I get an AirDrop dialog box that just says, "To share with someone using a Mac, ask them to go to AirDrop in the Finder." This dialog also has a clickable "Don't see who you're looking for" link. When I click on it, I just see a popup message: "To share with someone using a Mac, ask them to go to AirDrop in the Finder." (Sound familiar?) I do not see any way to point the AirDrop dialog at my iPad. So I just have to click on Cancel in the dialog box. That's my basic problem.

If instead I open the Numbers app on my iPad, I do see a browser that lets me look at various "Locations," including iCloud Drive. Tapping that location brings up four folders: Desktop, Documents, iMovie, Photos. Tapping Desktop shows me icons for all the items on my iMac Desktop. One of those items is Mag. Subscriptions on Desktop, the Numbers document I actually want to see on my iPad. Tapping its icon in the Numbers "iCloud Drive" browser on the iPad brings up the actual Numbers spreadsheet as it exists on the iMac, so this method is indeed a viable workaround!

But I have two questions:

One, why can't I actually initiate the direct use of AirDrop to share an open Numbers document from my iMac to my iPad by using the Numbers Share menu on the iMac itself?

Two, if I'm using the iPad-based workaround I just described, why don't I seem to have "iCloud Drive" access — via the browser in Numbers on the iPad — to all of the folders on my iMac, not just Desktop, Documents, iMovie, and Photos? The original of my spreadsheet document is in the folder hierarchy Eric's iMac>Macintosh HD>Users>eric>eric's docs>Portmanteaux>General>Consumer Items, but using the Numbers "iCloud Drive" browser on the iPad, I don't see any way to get at my original document within that folder hierarchy.

What am I missing?

Thanks in advance ...