Geting rectangles instead of entered text

I was trying Demo application of Radaee PDF library for Android: PDFReader. During entering text and numbers to form inside PDF, I see just rectangles inserted. After saving pdf document, I see this rectangles in other PDF applications too. Is this bug, or am I just doing something wrong ? Our company is interested in purchasing Premium licence, but this issue is critical for us. Please help me solve this.

It seems a font's issue.
What device and operating system are you working on?

Look at www.radaeepdf.com/support/knowledge-base...component&kbartid=25 , I suppose you could find it useful.

I have tried it on tablet LenovoPad_A10-70, Android 5.0.1, and on Emulator with Android 5.1.1.

I just have tried this solution, but it is still not working. I am not sure it is font problem, because I have tried many pdf files with different fonts. When I am entering values inside form field, I see characters and numbers, but when I than click on another field, i see only rectangles. Can you tell me, what else can be a problem here?

I'm quite sure the issue is fonts related.
May you provide one of your pdf file, please?

Adding DroidSansFallback font finally worked. Thank you for your help