Thank you for your ticket.
Unfortunately Joomla's latest versions have made some fundamental changes to how it's file filtering system works, which is why Ark Media has also made changes to keep up.
Firstly, if you have the latest version of Joomla and Ark Media then please can you check that
pdf files are still set as valid upload files in Ark Media's configuration?
If that check passes then your server may be missing the necessary libraries to now handle the
pdf files as Joomla has significantly altered how file uploads are validated now.
You can use your Joomla
admin/System/System Information to check whether you have
FInfo installed, as this is the required PHP library that Joomla is dependent on. Otherwise you will need to ask your hosting provider to install the FInfo PHP extension, as Joomla is now dependent on this extension.
If you run these two tests for one of your sites and find what the issue is, it is likely that it will be the same for all of your other sites. It is likely to be the missing PHP library on your hosting environment which can be fixed quite easily. However, if it is a incorrect Ark Media configuration, let us know and we'll see if there is a way to speed up fixing all of your sites faster than manually going through each one!