I would like to produce Prince PDFs that open in Adobe Reader at 100%.
Adobe's documentation states it can be done with a URL, e.g.
some.pdf#zoom=100
Can this be accomplished for some.pdf with a Prince CSS rule?

We don't support this feature in Prince; it sounds like something that could be added to the @prince-pdf rule that we have discussed on the roadmap.

In Prince 12 we have supported this:

@prince-pdf {
    prince-pdf-open-action: zoom(fit-page)
}

You can also specify fit-width or fit-height or a percentage.