The API aborts a conversion if it takes more than 40 seconds. In your case, it takes more than 70 seconds just to fully load the source HTML document - that's why you are getting the timeout error.
Most of the time is spent on /assets/php/printbarcode.php?code=nnn. I think you have the following options to resolve this:
1/ Make the printbarcode.php handler faster.
2/ Or serve the barcode images from a warm cache.
3/ Or split the HTML document into several smaller parts, create PDF for each part separately and then join the PDFs together using e.g. pdftk (a free tool - http://www.pdflabs.com/tools/pdftk-the-pdf-toolkit/