Since the Raspberry Pi uses an ARM architecture you are most likely out of luck as the DLL SDK is probably x86 architecture only.
But don't give up hope. You still have more options:
You may be able to use a Minnowboard and get x86 Linux with Wine running under it and make use of the DLLs that way. I believe the Wine COM port code is pretty stable.
If it is truly an RS-232 you should be able to hook the reader up to the serial port of a Raspberry Pi, set your com settings ('baud', stop bits, parity, handshake) with stty and read the output of the reader in Linux. It should be a simple dump of the RFID interrogations.
You may need to do a little bit of protocol reverse engineering, but for a RFID reader this should be pretty easy (about as easy as it gets). If you need to, you might be able to dig up some stuff on-line or ask the manufacturer for the device communication protocol specs.
Here's a link to a serial library for the Raspberry Pi in straight C that should get you going.