What model and firmware version are you using?
With the 2.1.0 firmware, the sending XML limit is 6MB for FF130,FF230 models and 11MB for FF430 and FF830 models. Keep in mind this limit is applied to the entire request posted to the web server (so effective file size limit is smaller due to base64 encoding with inline attachments).
Unfortunately, the “/ffws/v1/data/userspace/admin/report.pdf” part of the example references a feature that wasn’t fully finished/supported but managed to make it’s way into the documentation. The feature was to allow the ability to upload and fetch files from the FaxFinder through the web API.
It is partially functional though and here are two ways you can use it. The second method is what you are referring to.
1. Use the full HTTP URL to a file hosted anywhere the FaxFinder can fetch from (this is probably the recommended method if this works for you).
<url>http://example.com/myfax.pdf</url>
2. Upload the file as mentioned using scp to the following location on the FaxFinder, assuming you are logging in as admin to post the XML: /usr/local/ffweb/data/userspace/admin/myfax.pdf
And then set your url to:
<url>/ffws/v1/data/userspace/admin/myfax.pdf</url>
Hope that helps.
Jesse