I'd just been looking for some Japanese tutorial PDFs, and there are some really good ones. Like the Japanese Cheat Sheet and Tofugu's Japanese Particles cheat sheet and the PDF version of Tae Kim's Japanese Guide to Japanese Grammar.
If you've been trying to check out these, and other similar PDFs in Ubuntu, you may have noticed that any Japanese symbols aren't visible. Basically this is because the appropriate fonts aren't available. Fortunately there is a very simple solution! Simply install "poppler-data"
sudo apt-get install poppler-data
Either you can use the command above, or use your preferred package manager. If you check out the description of it, you should see something like "If CJK users want to view PDF files that contain their language...". It may not be immediately obvious, but CJK stands for Chinese, Japanese and Korean.