Where to upload
Any place that asks for a deck opens the same upload dialog, and a deck uploaded anywhere is the deck everywhere in your project. You do not need to upload it more than once.
What happens to it
Every upload is scanned for malware before it is stored, and stored encrypted. Roundly extracts text from it to label the claims on your profile with where they came from.
Who can see it
Your deck is private. Investors see that a deck exists as an evidence source; they can request access, and you decide each time.
If the upload fails
A deck that cannot be scanned is refused rather than stored unscanned. Password-protected and unusually large files are the common causes — export a standard PDF and try again.