To have Dimension automatically convert SINGLE page PDF files to TIFF files, follow the procedure below. 

• Click on Settings/User Preferences/Files (Ribbon Bar)

OR

• Click on Admin/User Preferences/Files (Classic Toolbar)
• Check the "Automatically convert PDF files to TIFF files box

PDF files will be automatically converted to TIFF files when each file is first opened.Warning: TIFF files are bitmaps and will become pixilated (distorted) when zoomed in very closely. Most PDF files used for construction drawings are also bitmaps and suffer this same distortion. But some PDF files contain CAD commands and suffer no distortion. In this case, the PDF file may be preferable to the TIFF file.