This issue arises when you copy paste text from other programs like word or outlook that contains illegal HTML or Unicode characters. The simplest way to resolve this issue is to find which section the PDF output is getting stuck at by trying to print each section of the document until you find the issue.
Once you've found the problem page:
Copy the content from the text boxes on that page.
Paste the copied text into a program like Notepad to cleanse the formatting.
Save and reuse the cleansed text as needed.
While this is a simple workaround for now, we are actively developing a script to automate the process and exploring ways for Mosaic to handle it independently in future versions.
If you require any further help, please raise a new case on the customer success portal.