The Chinese and Japanese languages are the only two languages in the world that don't use spaces to separate words. Instead, they just stick all words together. So it's common to see a whole Chinese/Japanese paragraph without any spaces in it.
Because of this peculiarity, MediaWiki's "Create a book" and "Download as PDF" features don't wrap Chinese/Japanese lines in a generated PDF file, resulting in a whole paragraph rendered on a single line and truncated when the line runs out of the right edge of the page.
HOW TO REPRODUCE THE PROBLEM:
- Go to http://www.mediawiki.org/wiki/MediaWiki/zh-hans
- Open the page's "Download as PDF" link in a new browser tab;
- You will get a PDF file that doesn't wrap a long Chinese line but instead truncates it at the right edge of the page:
HOW TO FIX IT
- As a quick workaround, consider inserting special, invisible Unicode control characters into a long Chinese/Japanese line that will cause line wraps;
- As a dirty rule, if you can't wrap a line at a space, wrap it at the right margin of the page forcibly;
- In principle, every Chinese/Japanese character can be considered a "word" and therefore a line wrap is allowable before or after any Chinese/Japanese character;
- Experts may have a better solution than the above.