When editing a Custom Template Content section, SteelBrick best practices recommend Cloning the Template Content and then editing the clone. This ensures you always have a working Template content section reference and don't run the risk of editing your already-working content in a way that would make it unusable.
This error often occurs due to changes in the row and column layout of the HTML table that holds the custom Template content.
Users may also see the Failed to Load PDF Document error due to unintentional field value changes:
For some users, migrating from Sandbox to Production using the Data Loader will cause field values with more than one 0 to be truncated to a single zero. This is most common for color field values written in color hexidecimal, i.e. 000000 for black:
This is an Excel and Dataloader issue unrelated to SteelBrick functionality. However, you can work around this by ensuring your cells in Excel are formatted as text and not as numbers. You can also manually change any already-truncated numbers on the Quote Template.