PDFs not airdropping to iPad

I am trying to send a number of PDFs by air drop from my iMac to my iPad. I do this rarely and it feels like it used to work. The documents are sending ok and the iMac panel confirms they sent fine. I get a panel on my iPad telling me the document has been airdropped although it only flashes up for less than a second. However, I cannot locate the documents either in the Blue Desktop "Files" Folder, nor "on my iPad", nor in "Recents" nor in "Shared", nor in the "iCloud drive" folder, nor in "Photos". Using the search bar and entering a document title it find nothing.receipt. I have restarted both machine and switched bluetooth off and on again.

If I airdrop a photo it works fine and immediately opens up the photo on my albums page. If I airdrop a text doc or a screen snap shot, Png, Jpeg it works just fine and drops all of those into the photo folder.

However it will not appear as a PDF doc anywhere. What on earth is going on?