For PDF templates, you can use another app or website like this one that converts it into a Word Document and then you can upload your word document to Google Docs.
If your word doc template is complex with a lot of formatting and images, it may help to start with fresh google doc and copy section by section or insert same images that are being used in the word document to ensure there is no hidden formatting when using template that could possibly cause any inconsistencies or formatting issues later when generating documents.
Kindly note that it can take time to recreate your Word or PDF template into google docs format and the document may not look the same in this format. It could help to start with fresh google doc rather than uploading an existing template and build from there as Google Docs has many great features for formatting for making it look nice and see this article to learn more about formatting with Google Docs.
You can this article for an option for how to convert a PDF file to google doc format.