Welcome to the community @Rodrigo_Meyer!
Probably best if you build your own custom image based on the one of n8n.
So you create a file named “Dockerfile” with this content:
FROM n8nio/n8n
RUN npm install -g easy-pdf-merge
In the same folder you execute then this command to build it:
docker build -t n8n-pdf
If you then use this image and start n8n like this:
docker run -it --rm --name n8n -p 5678:5678 -e NODE_FUNCTION_ALLOW_EXTERNAL=easy-pdf-merge -v ~/.n8n:/root/.n8n n8n-pdf
the library should theoretically be accessible.