I have an Illustrator file and I can save as PDF, but need to reduce the size of the resulting PDF. What's the best way to achieve that?
Are you using any photos in your AI source file? If you do, when saving to PDF look for the 'Compression' tab in the options panel that pops open. Either choose the 'Smallest File Size' preset or play with the compression options until your PDF gets reduced to the size you need.
I'd be careful using the Save As PDF settings to obtain the smallest file size without careful watch of image quality. It can easily degrade resolution below acceptable levels. Photos, gradients and shadows are lost first and text can be made unreadable.
If your file size is too large I suspect you have photo, gradient and shadow content. Compressing these things is not Illustrator's specialty.
I have 2 suggestions for achieving crisp and low size .PDFs from Illustrator:
Compress your images first. A program like Irfanview or Photoshop can take a 5MB image to 500kb or less with little appreciable loss in quality. Irfanview even lets you select a target file size, just remember to check quality. Replace the photos in your .AI file with the reduced versions, or just start with compact images.
Use File>Print and for printer select "Print to .PDF" This does a good job of preserving your raster art and keeping it to a reasonable file size.