Solution: Using PhantomJS made me able to render my own SVG-code and save it as a .pdf file.
Javascript:
var svgString = "<svg xmlns='http://www.w3.org/2000/svg' version='1.1'><path d='M0,0L600,200L0,200z'></path></svg>";
var page = require('webpage').create();
page.content = svgString;
page.render('aRectangle.png');
phantom.exit();