Hello,

According to this link: https://github.com/wkhtmltopdf/wkhtmltopdf/issues/4957 , you can resolved this issue by increase  file and stack limits with command: "ulimit -n 10000".

Or if you have interest, you can check out our product: Local PDF Report Generation and Printing, which offer the ability to generate and print from local PC, and reduce your sever load at the same time.

Best regard.

I had the same problem, I have solved it for some time by increasing the number of workers in my configuration then I faced the same issue with huge datasets. 

For this, I have solved the issue by adding queue_job module from OCA.

I hope this helps!