If you are going to use it for Flex, you need a true discriminator tap which the pc/if output is not as K9WG pointed out.
Second, for any decent decoding, you need a hardware level convertor and a true serial port on the computer. For Flex, you need a 4 level interface. USB to serial adapters do not work either.
Then I often wonder what there is too monitor that is legal? Monitoring your common public paging systems is against federal law.
We own a pocsag system at work and I do use PDW for troubleshooting that system but it is my system and license. For decent decoding of my systems signals, I do need a hardware interface as the soundcard input is iffy. Our system is also very low power and intended as an in house system only so not much signal when a user gets more than a mile or two away.
If I'm not mistaken, the Uniden HP-1 with all the latest upgrades and firmwares installed can output a true discriminator level audio signal for decoding. I have never tried it though so I can't say how well or if it works.
edit: you may get some decoding of low speed (512 baud) pocsag systems straight from the external headphone jack but the 1200 and 2400 systems will need true discriminator level audio.