Several reasons trigger this problem, firstly, the QB print.QBP file is damaged or missing, Adobe reader is not set as a default PDF viewer, you might have installed an outdated version of Adobe reader and lastly, the QuickBooks version you are using is outdated. To resolve QuickBooks unable to locate PDF viewer, you will have to take a few specific measures. Firstly, you will need to rename the QBprint.QBP file and add old in the name. Another fix is to update or reinstall the Adobe acrobat using an installation disk or directly from the website. Also, make sure to set Adobe as a default PDF viewer, so that the problem should not haunt you in the future.