Since 1.8.0 extension version you can translate the labels via editor. This article is for older extension versions.
1. Copy the file
app/code/Magetrend/PdfTemplates/i18n/en_US.csv
to
app/code/Magetrend/PdfTemplates/i18n/de_DE.csv
where de_DE is your store locale code.
2. Translate it!
3 Clear the cache by running the following command via ssh
php -f bin/magento cache:flush translate
or via Magento admin side: System >> Cache Management