You dont have to do anything different to decompress.

In workbench, what is your output variable on the Barcode.Decode step?

Is it defined as xml?

Did you print+scan your input file?  The Barcode.Decode can't read the barcode from a interactive form.  It must be flattened, no xml hierarchy.

What does the adobe.log say when you run it?

binary data passed in a xml is encoded in base64 but i dont think thats your problem.  If it is, then you need to convert it back to binary.

I dont know c#, but i did find this..

Syste.Convert.ToBase64String(toEncodeAsBtytes)

System.Convert.FromBase64String(encodedData)

I think your problem is that you dont know how the data was compressed in the barcode.  In otherwords, if you created the pdf form in designer and checked the compress data parameter then the LC Barcode.Decode will be able to decompress it.  If some other program compressed it, LC Barcode.Decode isn't going to know what compression algorithm was used to compress it, so it wont be able to decompress it.

Take a look at this website

http://www.motobit.com/util/base64-decoder-encoder.asp

Is the content of your binary in characters alphanumeric?

/9j/4AAQSkZJRgABAQAAAQABAAD//gA8Q1JFQVRPUjogZ2QtanBlZyB2MS4wICh1c2luZy BJSkcg

SlBFRyB2NjIpLCBxdWFsaXR5ID0gMTAwCv/bAEMAAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQ EBAQEB

AQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAf/bAEMBAQEBAQEBAQEBAQ EBAQEB

OR is it a bunch of letters, and symbols?

ÿØÿà  JFIF          ÿþ <CREATOR: gd-jpeg v1.0 (using IJG JPEG v62), quality = 100

ÿÛ C                                                                 ÿÛ C                                                                 ÿÀ    X   "       ÿÄ                            

ÿÄ µ                }        !1A  Qa "q 2‘¡ #B±Á RÑð$3br‚

     %&'()*456789:CDEFGHIJSTUVWXYZcdefghijstuvwxyzƒ„…†‡ˆ‰Š’“”•–—˜™š¢£¤¥¦§¨ ©ª²³´µ¶·¸¹ºÂÃÄÅÆÇÈÉÊÒÓÔÕÖ×ØÙÚáâãäåæçèéêñòóôõö÷øùúÿÄ                 

The 1st one is base64.  The 2nd is binary.