This is the 10340th most frequent Russian word.
бумажную
"Paper" (adjective form).
In this sentence, 'бумажную' specifies that the book is made of paper, contrasting it from digital formats.
Она взяла бумажную книгу со стола.
She took a paper book from the table.
Here, 'бумажную' describes the physical composition of the napkin as being made of paper.
Я написал заметку на бумажную салфетку.
I wrote a note on the paper napkin.
In this context, 'бумажных' describes the material of the boxes, implying they are made from paper.