Do many Germans actually say that?
It seems a bit mad to me to just import English words like that (the two parts of ehich have no meaning in German) when they have a perfectly good German word already. It reminds me of when I saw "C'est easy" on a poster on the Paris Metro..
It's likely that Anhang is an attachment to an email.
But I suppose theoretically you might download an appendix to a book that is available for download as one PDF file per chapter -- and you're downloading specifically (say) Appendix C.
I'll add that possibility.
For the same reason that we don't "load under" or "underload" something but instead "download" it.
"under" just indicates location while "down" indicates direction of movement (going down the stairs, falling down, etc.). Compare also "He jumped under the stairs" and "He jumped down the stairs".
Similarly with German unter (location) versus hinunter, herunter (movement).
Here, herunter is used because the movement is towards the speaker rather than away from the speaker.
Yes, hoch "up, upwards" indicates movement. The corresponding location is oben "at the top" as an adverb, auf "on, on top of" as a preposition.
There is no hinhoch, herhoch, hinoben, heroben.
However, hinauf, herauf are possible and also indicate movement upwards.
hinauf, heraus, hoch are more or less synonyms.
Ladt and herunter both means download
Not individually, only together.
lädt ... herunter together translates to "downloads".
Kind of like how "She is looking that word up in a dictionary" requires both "looking" and "up" to convey the meaning "search" -- the meaning doesn't come from either "looking" or from "up" alone.