It's a colloquial expression for "ingoiare" (to swallow); "it is difficult to send it down" would be "è difficile mandarlo giù" but "it is difficult to send down" would be just the same "è difficile da mandare giù". Note that the sense here isn't that I'm having trouble swallowing, but that the object I'm talking about is in itself hard to swallow, most likely in a figurative sense (it's hard to accept).
No, it does mean swallowing, although the figurative sense is more common. It has a more negative sense than its synonyms inghiottire, deglutire, ingoiare and ingollare, as it refers to swallowing without tasting, e.g. "manda giù la medicina" (swallow the medicine). Niccolò Fabi wrote a song that went "Bevo la mia acqua e la mando giù", "Bevo questo vino e lo mando giù", "Bevo il mio veleno e lo mando giù" (drinking and swallowing in order water, wine and poison).
reverso.net's context search shows that this has nothing to do with actually swallowing something, like a piece of food, but is totally idiomatic, meaning "it's hard to accept".
Il nucleare è difficile da mandare giù dopo il Giappone.
"Nuclear energy is a tough sell after Japan."
Comandante questa spedizione è difficile da mandare giù.
"Even so, Sir, this expedition is going to be hard to swallow."
For food: inghiottire il cibo = "to swallow the food"