You know, in the Android Electrum Send tab there is a button (diskette) which is a Save transaction button indeed, I think.
When I press it Electrum says 'invoice saved'. But where is it saved , no idea.
I thought it was a sort of the Desktop's Export button (which can be used to save and pass transactions from Desktop watch-only wallets.
It's not. When you save an invoice, they are stored on the "Invoices" page so you can send them later. When you click on the invoice you want to complete, it will fill the amount and address fields so you can send it. That's all. You can test yourself by checking the "Invoices" button right next to the "Save" button. Like I said, (AFAIK) there isn't any way of exporting the transaction to a file.