I realize this Question/Answer is from several months ago, but I just ran into an issue myself that I am guessing Robincnak may be referencing. The issue is when you are on a Website that has a Webform with an 'Add' button, for example, and then you select 'Browse...', which opens the iOS 'Files' app. You can navigate to locations such as iCloud or even Dropbox, etc.—whatever Locations are listed in the left pane. When you view the contents of the Location and any subfolder, etc., PDF files are 'inactive' or grayed out. Image files are NOT grayed out, so you can upload those.
Someone told me it is an iOS limitation that will not allow you to upload PDF files, which I find surprising, but...maybe it is...?