You can connect the pdf file to an object in AutoCAD
using the HYPERLINK command.
Just enter the HYPERLINK command and select an object,
a dialog box will be opened and you can select the pdf
file using the "File..." button.
Click OK.
to recall the pdf file select the related object and
right-click. From the context menu select the command
Hyperlink -> "pdf file name"
The pdf file will be opened.