Put a 100n cap (10n is most often enough, but 100n is often a handy BOM reuse) from signal to ground directly at the ADC input pin. Provides RC filter for noise & the required low impedance for the ADC sampling. A great low-cost alternative to the "standard" opamp follower circuit, when you don't need high sampling rate (so that the cap has time to replenish through your high resistance between every conversion).
Although they say that PIC ADC works acceptably with fairly high input impedance, many SAR ADCs don't work well at all with 10k input impedance. Often, well below 1k is desirable, hence the opamp buffer or the simple capacitor.