Hello Jeffrey,
I am sorry about the issue you are experiencing and I am happy to assist you with this.
Just to confirm, you disabled the W3 Total Cache and the issue was gone?
Can you please share which plugin you are using for the PDF vouchers?
Also, please disable the settings one by one in Performance>general settings, save the settings and purge the cache after each setting is disabled, and see which one might be causing the issue
Thanks!