How to connect USB barcode scanner to a PIC?
You would be better off with a barcode reader with serial or PS/2 interface. Sometimes it is as easy as swapping the cable for a different one to change the interface type, depends on your barcode scanner model.

If you really want to use the USB barcode reader, you need either


a PIC that can be an USB host like PIC24FJ256GB106 or PIC32MX420FxxxH or
a standalone USB host controller like MAX3421E
a software implemented USB host stack - I know they exist for Atmel, not sure about PIC


Even with these hardware solutions it would still take some effort to implement enumeration, the HID device class and so on.

Outside the scope of your question and probably more expensive than a serial or PS/2 barcode scanner, but there is a precompiled firmware for FTDI's Vinculum chips that bridges an USB HID device to a microcontroller's SPI interface, see USB HID to SPI Master Sample Application ROM.