Hi nathanielj4220, As per the issue description mentioned above, you are getting above mentioned error message when trying to open pdfs, is that correct? Can you start up Reader after you get the error? If so, go to Edit>Preferences>Security (enhanced)>Sandbox protections and deselect Enable Protected Mode at startup and enhanced security. See if that helps. note: Disabling protected view/ Enhanced security is for troubleshooting purpose only, and not recommended. Please turn the feature back on after troubleshooting is complete. If issue still persists, try repairing reader using "Repair installation" option under Help menu ( for Windows OS only), reboot the machine after repair and check. You may also refer to the following forum threads discussing the similar issue: Application Error message (0xc0000409) occurred in the application at location 0x00411e0b (Installation & updates) unknown software exception (0xc0000409) occurred in the application at location 0x0041240b Also, let us know dot version of reader and the operating system installed on the machine? You may refer to the steps given in this link on how to check the version in reader:Identify the product and its version for Acrobat and Reader DC Let us know if you need any help. Shivam
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