Hi Axel, there are 2 different modes for the PDF export.

One mode is if you have any solids or surface objects included as the objects being written. In this situation MoI will generate a hidden line drawing and there will be various kinds of curves generated like hidden lines, silhouettes, and outlines. There will also be a larger options dialog shown to control these generated curves.

The other mode is if you are doing a wireframe only export where only curve objects are being written to the file and not any solids or surfaces. In this mode there will not be any hidden line drawing generated and the options relating to the hidden lines will not be shown.

There is not currently any option to control the line width for a wireframe only export.

So probably when you're exporting selected objects you've got only curves selected and so that's making a wireframe export and when you do a "Save As" where all objects are being written you've got some solids or surfaces being processed there for making a hidden line drawing.

- Michael