The “problem” is that Illustrator is not a PDF editor:
Prepression: The Ten Commandments of PDF
Use a PDF editing tool such as Enfocus PitStop Pro.
Now that I have that out of way…
Two other options include:
1) Acrobat Pro – Tools/Document Processing/Export All Images (double check the settings)
2) Photoshop – Open the PDF, select the “Images” button (not pages), then open all of the images used in the PDF and save out for linking in Illustrator.
If you are going to use the optimised save, be careful not to recompress or resample.
Stephen Marsh