@xinzheng

To install wkhtmltopdf on Ubuntu 18.04 droplet please execute these commands

sudo apt install xvfb wkhtmltopdf

Then use this command to run it

xvfb-run -a --server-args="-screen 0, 1024x768x24" /usr/bin/wkhtmltopdf

Pass arguments at the end.

Hope this helps, best of luck from DO community :)