Salesforce CPQ error ‘Failed to Load PDF Document’ with Quote Template Documents

First, determine which section is causing the error.  Second, fix the error and then test the template.
A list of items that you might check to troubleshoot the issue---

Quote Template Detail

Check all color fields on the Quote Template Detail to make sure their values are a valid Hexadecimal color code. Using Data Loader to migrate from Sandbox to Production can cause a value of "000000" to be saved as "0". Also check all color fields in all template sections by clicking on the Section Name in the Sections related list on the Quote Template record. If this was caused by a data migration using a spreadsheet and Data Loader, you may be able to prevent this by ensuring that your cells in the spreadsheet are formatted as text and not as numbers. 

Determine Which Section is Causing the Error

1. Clone the Quote Template using the Clone with Related button (may have to be added to the layout). Always work with a copy to maintain your original as a backup.
2. From your cloned template, remove one section at a time until the document renders. 
3. The last section removed is causing an error. There may be other sections with errors that were removed previously, but this has to be done one at time.
4. Add the last section removed to a brand new Quote Template as the only section. Use this for testing.
5. If none of the sections fail individually, repeat the steps with content in the Header Content and Footer Content

Fixing Common Template Section Errors

1. If the problem section is of type Line Items, recreate it from scratch making sure all custom field API names are spelled correctly. Also, ensure there are no null values for fields referenced in the line columns.
2. If the problem section is of type HTML, go to Template Content and click on the name of the cloned Template Content record.
3. Click Edit to open the content editor.
4. Delete Tables, Rows, or Columns one at time, save and test by previewing the template to try to identify which section of the code is causing the error. 
5. To recreate your custom template section use the Template Content editor tools. Don't paste from a formatted document source like MS Office. Not all HTML tags and formatting is supported. Please see this article to know more about Supported tags.
6. You may need to log a support case so we can further assist in troubleshooting.