Hello,
I can write you module that will take care of your issue.
I suppose your documents are listed in prestashop as virtual products. I don't want to write too much of details, but i can say my module will link 10/100 version product together, so if the 100 version is sold thanks to my module, both versions will be disabled.
What prestashop version do you have ?
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I work daily with prestashop over 6 years. I know prestashop from top to bottom. I am familiar with every aspect of this eshop system. From adding simple features, theme modifications, creating custom prestashop modules to overriding core files and connecting prestashop to various API's.
Thank you for your time.
Kind Regards,
Jarda Nalezny