Need to download Monthly Payslip as single PDF for 400 employees in odoo 12 CE.

We had tried to download directly from the screen by selecting these 400 items from the list view and clicked "Payslip" download option under print.

Am getting different issues like

1. Not downloading all 400 pages but it is downloading pages from 100-200 max but in adhoc manner. Not able to identify the exact issue

2. Sometimes only one page is getting downloaded

3. Wkhtmltopdf failed (error code: -11). Memory limit too low or maximum file number of subprocess reached. Message : b''

Any suggestions / inputs on this please.