PIC24F08KL201 - PUMPEN Or Not PUMPEN?
This is a new one on me.  I hadn't heard of a charge pump built into a PIC before.  Unfortunately "charge pump" does not appear anywhere else in the datasheet except in the description of that bit, so I don't know what the charge pump is supposed to be for.

It's interesting to note that there is no mention of "charge pump" or the PUMPEN bit in the A/D chapter of the family reference manual, although that is indeed the specific document called out in the datasheet.

I think Microchip screwed up somewhere.  Either a new section didn't make it into the latest documentation, or something was cut and pasted from elsewhere and something got missed in updating it for the new purpose.  Microchip documentation is generally very good, but stuff happens, and sometimes you have to use a little intuition to figure out how the part really works.

If you don't know what something is and that you actually need it, keep it off.

This is also worth asking directly of your Microchip FAE.  This is a very legitimate question to put to them.  You should not bother them lightly, and do your part to RTFM carefully before asking, but when that fails go and ask.  If this is really a documentation screwup, as it seems, they need to know about it too.  Just be polite and realize that documenting all the nuances of a microcontroller is no trivial feat, and they can be forgiven for missing something on occasion.