No, the documents are not backed up when you back up QB.  The attachments live in a folder on your computer, you need to have backup systems in place that back up the files on your computer in order to save those documents.

Resolution for 'back up QuickBooks issue: 'Do attachments added to QuickBooks 2014 get backed up when you back up QuickBooks?' available: Yes.

The back up QuickBooks issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the back up QuickBooks issue yourself by looking for a resolution described in the article. If it is a complex back up QuickBooks issue or you are unable to solve the issue, you may contact us by clicking here or by using other back up QuickBooks support options.

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