Unfortunately the internal oscillator can't be boosted by the PLL to achieve 16 MHz. Its maximum speed is 8 MHz.
For 16 MHz, you still have more than one options:
Without using PLL, you can use 16, 32, 48 or 64 MHz XTAL directly, by adjusting the oscillator postscaler to 1, 2, 3 or 4 respectively. This done by the configuration bits CPUDIV0 and CPUDIV1 during the programming of the device. Also, "HS Oscillator" option must be chosen by the configuration bits FOSC3:FOSC0.
With using PLL, you can use 4, 8, 12, 16, 20, 24, 40 or 48 MHz XTALs. As the PLL only accepts 4 MHz input, PLL Prescaler configurations bits PLLDIV2
LLDIV0 must be configured accordingly during the programming. Also, 96 MHz output of the PLL must be divided by 6 to achieve 16 MHz clock, which is done by adjusting the PLL Postscaler via CPUDIV1:CPUDIV0. And of course, HSPLL or XTPLL option must be chosen by the configuration bits FOSC3:FOSC0. Don't use XTPLL unless you have a 4 MHz XTAL.
For more information about the configuration bits, see the "Special Features of the CPU" section in the
datasheet. Also the Figure 2-1 in the "Oscillator Configurations" section summarizes the possible configurations.
Note: I'm not sure if the XTALs I mentioned above are available on the market. But I guess it shouldn't be a problem to find 4 or 16 MHz ones.