Its recently came to our attention that many scanners come with software that scans old drawings to a PDF, and that users are doing just that. The follow up is how to put that in AutoCAD.
This is a waste of time and effort. The most efficient way is to scan to a raster file and use AutoCAD's IMAGE command to attach the image to the drawing. Avoid using JPG format, while it works well for photos it does not work well with line drawings. Use the PNG format if available, or use TIF if not.
Some users are reporting slow performance when they use the new PdfUnderlay functionality in 2009 (w/subscription) and 2010. It mostly depends on the content of the PDF, which can often be a bloated mess. Not all PDF creators are equal. For example, while one driver may write out text as a lean text element, another may (especially if its width factor is changed) write out each letter as a large collection of filled triangles. Take for example this innocent looking letter "S", which contained 471 objects!
AutoCAD doesn't know it's a letter S and has to treat it with as much importance as any other part of your file. This users file (PDF Size 1.2mb) contained nearly 325,000 objects that obviously was bogging down AutoCAD.
You can turn off the PDFOSNAP system variable to improve snaps.
Cannot Snap to PdfUnderlay
Not all PDFs contain geometry that AutoCAD can snap to. Many times the PDF is a scanned image that has such high resolution that the content looks like linework. You can usually zoom in tightly to begin to see the jagged edges of an image. Check your PDFOSNAP system variable; it should be set to 1.