Does node's exec call block meteor application?

I am building a meteor application that creates pdf files from a web page using phantomjs.

PhantomJS is called with node's child_process.exec function within a fiber using Meteor._wrapAsync. When I am using any url like google.com, it is working fine and the pdf is created.

The problem is, I want to capture a site served by the same meteor application at /invoicePDF. But during the execution of phantomjs the site apparently does not load, thus, it cannot load inside phantomjs, which makes finishing the call to phantomjs impossible.