Please, check the all Vtiger parameters: https://it-solutions4you.com/tipstricks/vtiger-system-requirements/
You can also try performing the following steps:
- Please download following zip file: http://its4you.sk/images/extensions/PDFMaker/src/ckeditor.zip
- Then go to the folder vtigerdirectory/modules/PDFMaker/resources.
- Rename folder ckeditor to ckeditorOLD.
- Open the zip file and copy ckeditor from zip file to this folder.
- Next go to the CRM, clear browser cache and try again.