Before looking into your case further, I want to let you know that Kintone does offer a native plug-in called PDF Preview now.
This plug-in lets users quickly click on PDF attachments and see their contents without downloading them. The PDFs can be previewed on the record/record list pages.
You can download the plug-in here:
Could you check this to see if this works for you?
FYI, you can find other native plug-ins here, too: