Regularly are issued "reports" on files that do not open. It is necessary to clarify 4 things:
01 - Despite the DXF format is the most popular, it is not the only supported file format (read the FAQ).
02 - "All" uploaded files are opened and checked before publication, which totally exclude the possibility of publishing files with error.
03 - PDF files can contain vectors and these vectors can be imported and edited in Corel Draw.
04 - The objects in the file are not always in the center of the page, they can be spread around.
Corel Draw is able to open/import all vector formats supported in the forum.
If you downloaded a file and not can open, try import to Corel.