We are looking to request/receive a PDF document via a webservice call, how do w

Have you looked a the Base64 Utilities plug-in? it has a smart service that will take a base64 string and convert it to a document in Appian which you can later display in a form.

As Eduardo pointed out above, you can build your web service to pass back base64 data (string) to Appian. Then use the smart service "Appian Document from Base64 String" which is available under Document Generation category after you install the Base64 plug-in.

Just checking, after using the above smart service, will the UIT having inline document work fine in Tempo?

Maybe you could build a plugin that converts the PDF to PNGs and show them in the UIT.

thanks for the response! I am in the process of tossing together a prof of concept process. I will update later and may include a version of the flow if it is worth sharing.

Ok, I've built a process and been fiddling with it, but I have not yet gotten it to work.
I have the string from the webservice isolated in a text PV, and am then running the decode function on it:
base64decode(pv!isolatedBase64_text)

When I write that value to a text variable, the header information looks good, so I do think the decode is occurring correctly. However if I pass the result to a Document variable, I get a -1. I have got to be missing something, any clues?

There should be a smart service that takes the base64 encoded string an creates a new document.

If you installed this plug-in forum.appian.com/.../summary then there's a smart service called "Appian Document from Base 64 Smart Service" (see attachment)

I am still not getting a document out of it, I am attaching a screen shot of my model.

Some datapoints:
the string in the response body comes out to about 44,000 characters, but when base64decode() is run against it the resulting string is just over 700 chars (seems short to me.) bring this up because the logs show a couple of errors:
first (the reson I think there is an issue with the input to [Appian Document from Base 64 string] is being truncated somewhere):
2014-03-11 13:02:48,627 [default-threads - 15] ERROR com.appiancorp.process.runtime.activities.custom.Base64StringToDocumentActivity - java.lang.ArrayIndexOutOfBoundsException: 65533
java.lang.ArrayIndexOutOfBoundsException: 65533
          at org.castor.util.Base64Decoder.translate(Base64Decoder.java:167)
...

then (I interpet this as just "it failed to create the document" likely due oto the prior error:
2014-03-11 13:02:48,635 [default-threads - 15] ERROR com.appiancorp.process.engine.UnattendedJavaActivityRequest - An error occurred while executing activity: id=3592, classname=com.appiancorp.process.runtime.activities.custom.Base64StringToDocumentActivity
com.appiancorp.suiteapi.process.framework.ActivityExecutionException: The document could not be created. Error: 65533
          at com.appiancorp.process.runtime.activities.custom.Base64StringToDocumentActivity.execute(Base64StringToDocumentActivity.java:45)
          ...

You have to put the base64 encoded string into the smart service. The base64decode function is of no use in this case.