Given that it fits, you have nothing to be worried about. You are absolutely required to have your passport with you and highly recommended to have your valuables with you, plus medicines and other essentials, so it is normal to use a carry-on or personal item (or both) to have these. Most laptops even fit in personal item sizes which are more restrictive than carry-on.
United is not strict at all about carry-on sizes and I have seen countless clearly oversized bags being taken into various short to long flights with them. On small planes they offer to take larger ones as gate checked luggage to those who have among the largest items, most typically roller bags.
While they do not actively measure carry-on size, there is a sizer at most gate entrances. This is essentially a metal cage the maximum size of the carry-on they allow. If your bag is suspected of being too large, you can be asked to place it in the cage. If it fits, then you are good to go. If it is not, then they can ask to convert it to checked luggage, in which case you may be charged an extra fee. You will usually be instructed to take out any valuable items and essentials.
A personal item is a good way of ensuring you will have your most important items with you and United, as the majority but not all airlines, allow passengers to bring one in addition to your carry-on. You can use pretty much anything as a personal item, even a folder as you suggested but I have been plastic and cloth bags as well as unpackaged items. It's a good idea to carry an extra cloth bag inside your carry-on that you can use to carry essentials in tight planes where even an allowed carry-on does not fit in some overhead bins and you end up in a seat without space under the seat in front of you or no seat in front of you at all.