Thanks for your answer. As a matter of fact the original PDF documents were created from an application using a free tool which configures a "printer" which prints into a PDF-file (and not onto paper).
As said before: the generated PDF document looks just perfect and as expected. Afterwards no changes are made to the PDF document. Seems to me that the application is the reason why the
resulting text documents are turning out different. Maybe some formatting or control characters are included.
But to understand you correctly: there's no solution to solve this issue.