If using curl, be sure to use the --data-binary flag (and not the -d shorthand) to avoid stripping newlines from the XML.
curl -X POST --data-binary @input.xml -H "Content-Type: text/xml" https://api.mc.chai.org/api/ValidateXml -D -curl -X POST --data-binary @input.xml -H "Content-Type: text/xml" https://api.mc.chai.org/api/XmlToPdf -o output.pdf