Presumably you are just pasting the graphic into the document. There are a few alternate methods to reduce the filesize:
Option 1: Use File → Place...
Instead of pasting the raster image into your document, use File → Place.... This will create linked files in your document:
It's important to note that your .ai file will no longer be self-contained. If you delete, move, or rename the source image, Illustrator will not be able to find it. This will not matter for the exported PDF.
Option 2: Use Symbols
Instead of placing the images, you could use Symbols. Just create a Symbol of the raster artwork, then drag it into the document as many times as needed:
Option 3: Print the PDF
If you have Adobe Acrobat (or some other sufficient PDF printer), you can print the PDF.
- File → Print...
- Select Adobe PDF as the Printer
This is the most efficient method and is okay for proofing. If the file is just being passed around to be viewed on a screen, this is usually fine. However, do not use this type of file for printing. All sorts of compression and nasty things happen when you print as a PDF.
Using my 2.78 MB example file, here are the results of the methods I just outlined:
- Copy and pasting: 46.1 MB
- Placing: 7.84 MB
- Symbol: 18.6 MB
- PDF Print: 139 KB
Note: These are all using the Illustrator Default PDF saving options. You could get better results by selecting another preset or modifying the settings yourself.
For example, the copy and pasting method saved with the [Smallest File Size] preset output for me as 150 KB which is comparable to the PDF print method.
- File → Place: Low PDF file size, but the source .ai will not be self containing.
- Symbols: Source .ai file will be self containing, but PDF file size will not be as low
- Print as PDF (or use Smallest File Size preset): PDF file size will be extremely small, but the resulting file is not sufficient for print