I work a lot with PDFs created with CAD software of different kinds. It often happens that while viewing the drawing in PDF XChnge Viewer 2.5, black frames appear around text and make the drawing less legible. They don't show however in Adobe Reader for instance.
I tried to hide all the layers but it seems that those rectangles are on none. I tried also to change the visibility properties of them, but those are disabled.
The Viewer is no longer developed and as such there will be no future updates and/or fixes for it. This product has been replaced by the PDF-XChange Editor.
This problem is fixed in the latest version of the Editor and you should not see these boxes in it.
If you would like to continue using the Viewer, please try to hide all comments instead of layers. You can do that from the main menu -> Comments -> Show Comments -> Hide All Comments (Ctrl+Shift+8).
These boxes are automatically generated by AutoCAD and they all have the same Author.
You should be able to hide only the comments from AutoCAD from Comments -> Show Comments -> Show by Author -> Select Authors.
