I have learned that, even one uses module-info.java to name its modules, on multimodule pom projects, childeren module names and the foldernames of the childeren has to be same.

There,

• As I have changed the name of the childeren project resides in openhtmltopdf-core\ folder to com.tugalsan.api.file.pdf.pdfbox3.openhtmltopdf.core, I had to change its directory to com.tugalsan.api.file.pdf.pdfbox3.openhtmltopdf.core as well.
• As I have changed the name of the childeren project resides in openhtmltopdf-pdfbox\ folder to com.tugalsan.api.file.pdf.pdfbox3.openhtmltopdf.pdfbox, I had to change its directory to com.tugalsan.api.file.pdf.pdfbox3.openhtmltopdf.pdfbox as well.
  
  P.S.: I do not know how multimodule project functions well. But I have learned that its groupId does not have to be different from my other projects. To simplify things I changed the groupIds to com.tugalsan; hence console message sizes shorten signifacantly.