This topic is well-explained in more detail in this  attached PDF article >>  ProtaStructure : IFC and DXF Import Guideline.

External Reference Drawing  

This function allows you to load DXF files and display the drawing as ghost reference layers on plan view or 3D view.

Considerations in importing DXF Files as External Reference Drawings :

The imported DXF files must follow the rules, else the drawings will not be displayed on plan view. Please read the rules below carefully.

1. The following restrictions and recommendations apply to the DXF Files to be imported as External Reference Drawings:
   
2. In order to increase the performance, all the unnecessary entities must be deleted from the drawing before it is converted into a DXF file and importing it, especially hatching, title blocks, logos, etc.
   
3. Ensure that all frozen layers are turned ON in order not miss out any layers.
   
4. Only entities below are imported:
   
1. LINE
   
2. ARC
   
3. CIRCLE
   
4. POLYLINE
   
5. LWPOLYLINE
   
6. TEXT

Therefore it is recommended that more complex entities to be deleted or converted to these object types before generating the DXF file to be imported.

1. To import the BLOCK entities, you need to EXPLODE them.
   
2. AutoCAD dimensions are also blocks. In order to import the dimension entities, you have to explode them to LINE or TEXT.
   
3. If you are using programs that generates special objects (like Autodesk ADT), apply “AECExplode” command before creating the DXF file.
   

   

   Note that, “AECExplode” command generates BLOCKs and these blocks must also be exploded using “EXPLODE” command.
4. For BLOCK contains Attributes Definitions, "BURST" command should use instead of "EXPLODE".
   
5. Ensure objects/drawings are not located far away from origin. (preferable to the positive quadrant with positive coordinates).
   
6. If the drawing is prepared using a rotated “World” coordinate system, then the drawing must be rotated back to the desired orientation before importing.
   
7. TEXT entities will be displayed with simplest font with minimum number of vectors to minimize the size of the DXF.
   
8. Completely switch off / freeze / delete all unnecessary entities such as hatching, title blocks, architectural objects, elevation views, etc from the original DXF before importing the file into ProtaStructure.
   
9. If there are several floor layout in a single DXF file, split each floor layout to different DXF files.
   
10. As a general guide, limit a single DXF file to a maximum of 5 MB.
    

Import DXF  

1. The file must be drawn in the units currently used in Graphic Editor.
   
2. Axes must be drawn using LINE entities.
   
3. All axes must be grouped in a single layer or two layers (for direction 1 and 2 axes).
   
4. Columns must be drawn using POLYLINE entities.
   
5. All columns and wall members must be grouped together in a single layer.
   
6. Ensure that the DXF format is not saved as Binary DXF format. If the drawing is Binary DXF format, you may change the format/version by "Save as" in AutoCAD.
   You may check the format by right click on the drawing > open with .. > Notepad. Example below:
   

You may check the Drawing format version codes for AutoCAD of your drawing here.