You reference and place underlay files in drawing files the same as you do raster image files; they are not actually part of the drawing file. Like raster files, the underlay is linked to the drawing file through a path name. The path to the file can be changed or removed at any time. By attaching underlays this way, you can use files in your drawing without greatly increasing the drawing file size. You can only view DWF and PDF underlays in the 2D Wireframe visual style. DGN underlays can be viewed in any visual style.
There are a few things specific to PDF files that you do not have to consider with DWF or DGN files. PDF files with more than one page are attached one page at a time (as opposed to one sheet at a time for DWF files or one model at a time for DGN files). Also, hypertext links from PDF files are converted to straight text and digital Signatures are not supported.
You can reattach an underlay multiple times, treating it as a block. Each underlay has its own clip boundary and settings for contrast, fade, and monochrome. However, you cannot bind an underlay to a drawing and you cannot edit or modify the underlay’s content.
If the underlay file contains layers, you can control how the layers display after attaching the file. If the file does not contain layer information, the Underlay Layers dialog box does not display any layer information.
For example, drawing A includes a DWF underlay showing some mechanical details. You need the content of drawing A attached to your current drawing, drawing B. If you attach drawing A as an external reference to drawing B, the DWF underlay that was already attached to drawing A is also be present.
If a file you want to attach as an underlay is password protected, you are prompted to enter the password after you have selected the file. You cannot attach the file until you have correctly entered the password.
After an underlay is attached to a DWG file, you are prompted for the underlay file’s password each time you open the DWG file. If the DWG file is also password protected, or has several other protected drawing references attached, you may be prompted for multiple passwords.