Thanks for taking the time to report this!
After "Add this page to your collection", what am I supposed to do in order to see the problem? Click on "Show collection" and then the "Download as PDF" button?
If those are the steps then I do not get squared but just empty space where Chinese letters should be in the PDF.

Also, on https://en.wikibooks.org/wiki/Chinese_%28Mandarin%29 , there is a box at the bottom saying "A PDF version is available. File size: 272KB (info)". That one does not show the problem for me.

Thank you for answering so quickly.

For generating the english version I proceed as follows:

• Enter https://en.wikibooks.org/wiki/Chinese_%28Mandarin%29
• On the left side I choose 'Create a collection'
• 'Start the collection creator'
• In 'Lessons' -> 'Introduction' -> choose 'About Chinese'
• 'Add this page to your collection'
• 'Show collection (1 page)'; enter some title/subtitle

On the right hand 'Download as PDF'

The resulting page starts with title/subtitle and the text
"The Chinese language (<blank>/<blank>, " and so on...

In the german version (produced using the equivalent steps) I am reading in the PDF:
"Die chinesische Sprache (<two squares each filled with a question mark>/<two squares each filled with a question mark>; Hanyu" ....

german version:

Yes, I know that the english page contains a downloadable PDF that doesn't show the problem. But I want to generate a german version for which I can not get a precompiled PDF.

Thank you for further assistance in advance.

This might be missing fonts on the OCG boxes...

As already announced in Tech News, OfflineContentGenerator (OCG) will not be used anymore after October 1st, 2017 on Wikimedia sites. OCG will be replaced by Electron. You can read more on mediawiki.org.

And i assume in Electron the same pages render just fine?