Since QB 2011 is not supported on a Windows 10 machine, this could be due to several things.  Are you using Adobe DC?  If so, try uninstalling that and using the older version, or a different PDF creator.  You might also want to check this:…

Resolution for 'Adobe DC issue: '2011 Purchase Order is not saving as PDF file' available: Yes.

The Adobe DC 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 Adobe DC issue yourself by looking for a resolution described in the article. If it is a complex Adobe DC issue or you are unable to solve the issue, you may contact us by clicking here or by using other Adobe DC support options.

For more information about resolving issues related to Adobe DC in Quickbooks/QBO or to get support for the issue '2011 Purchase Order is not saving as PDF file', click the link below: