Why can't I attach files to my Zotero Records?

I want to use the plug-in, Zotero Voyant, but I need a collection of Full-Texts attached to my Zotero bib records.

I have tried creating child items in the collections by dragging and dropping the pdf files from my computer to the related item, but nothing changes. Am I doing something wrong, do I need something else to help with this? Please advise, and thank you!
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