This is a solution using iTunes.
Import the .pdf to the iTunes Library. Then find the file, select it, and go to “File Info”. Under "“Options” select “Music” (even if it is already selected, select it again). Now iTunes treats the .pdf as a music file. You can label it the same way the album song files are labeled (Artist/Album Artist/Album Title/etc) and it will be stored in the iTunes Library folder under the artist and album. Roon will recognize it, too, showing a blue “.pdf” icon with the album which will open if you click on it.