Background: When right-clicking some PDF files, there will be an option "Combine files in Foxit PDF Editor". This article explains how to recovery it if the option disappeared.
Troubleshooting process
1.Open Installation directory "C:\Program Files (x86)\Foxit Software\Foxit PDF Editor\plugins" > Check if the plugin "ConvertToPDFShellExtension_x64.dll" exists. If not, there is something wrong for the installation package ,it is recommended to re-download the package and re-install again. 
2.Open Registry Editor >Navigate to HKEY_CLASSES_ROOT\Folder\shellex\ContextMenuHandlers\Foxit_ConvertToPDF > Check if the value is {C5269811-4A29-4818-A4BB-111F9FC63A5F}
If not,please set its value to {C5269811-4A29-4818-A4BB-111F9FC63A5F}.
Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Classes\*\shellex\ContextMenuHandlers\Foxit_ConvertToPDF > Check if the value is {C5269811-4A29-4818-A4BB-111F9FC63A5F}
If not,please set its value to {C5269811-4A29-4818-A4BB-111F9FC63A5F}.
Navigate to HKEY_CURRENT_USER\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion,please right click on CurrentVersion this folder there and choose delete option in the pop-up context menu to delete it.
3. Run cmd as administrator > Execute following command line to register the plugin.
regsvr32 "C:\Program Files (x86)\Foxit Software\Foxit PDF Editor\plugins\ConvertToPDFShellExtension_x64.dll"
After register successfully, please open Task Manager > Processes > Find "Window Explorer" > Right-click to restart then check if the option "Combine files in Foxit PDF Editor" shows normally.
