Since the version 3.3 of LibreOffice Portable Draw it's now possible to import and modify PDF documents. This is possible because of the already contained PDF Import extension (look at Tools -> Extension Manager). In older versions this extension had to be installed.
After I've imported a PDF file, I had to find that some symbols of the original document were not imported correctly. For example, arrows and check boxes were replaced by strange characters. Probably the above-mentioned extension is responsible for this error. Or knows someone a solution for this problem?