You would probably don't want PDF based report/print to be sent from the server. Here's an idea for similar requirement (I'm developing my own OpenERP backed POS based on wxPython):

• Use any network LPR capable POS receipt printer hardware and setup/enable CUPS on the server, setup the printer so you can send string from the server to the receipt printer through network (lpr). Also make sure to use "Generic" - " Raw print queue" as printer type.
• Create print_receipt_method in the 'pos.order' object which basically construct text based raw command string which will later be sent to the printer through the use of LPR via TCP/IP.

Another option if your printer is connected to Point of Sale terminal through USB/Serial/Parallel,etc (this is the approach I'm using right now):

• Setup CUPS and Generic/Raw Print Queue on the POS terminal
• Create get_receipt_method in the 'pos.order' object This method construct text based raw command string and returns it to the POS terminal, this might also possible to be done through the use of Aeroo Reports (generic/raw format).
• Write the result to a temporary file, 'lpr' the file to a local POS receipt printer. If you are on Windows, you can use some free tools which basically 'spools' any file to a printer.

Especially if your POS are handling high volume transactions such as supermarkets where service time is important, I would stay away from PDF generation. I also haven't got much experience with the OpenERP's web based POS, because I end up developing wxPython based Point of Sale interacting with OpenERP XML-RPC service, this gives me the following benefits:

Much greater control over:

1. user interface
2. sale flow
3. hardware devices (barcode scanner, magnetic card reader, secondary/pole display, receipt printers, etc)

I can easily access and use USB, serial, parallel devices, send signals through TCP/IP, and any other low level hardware action.

As for the barcode scanner, isn't it suppose to be just replacing the keyboard key strokes? Well, unless the serial/com interface.