This error indicates that there are bits of unexpected text in your content that can’t be properly displayed. These characters may have made their way into your Vellum file via copy and paste.
To fix, you’ll need to locate and and remove or replace any of these characters in your book. Often these will appear as boxes or other shapes:
If you have access to Adobe Acrobat, you can quickly find these with the Acrobat’s Preflight tools. Use the Verify Compliance with PDF/X-1a profile to find problems in your generated PDF.
If you don’t have access to Acrobat, or you aren’t able to find these problematic characters by scanning through your PDF, please contact us. Attach your generated PDF to the message and we can perform this analysis for you.
You can learn more about Vellum in Vellum Help.
Still have questions? Contact us.