I'm creating a postcard (6 X 4 in) using Adobe Illustrator CC. When attempting to upload it to the printing company, the .pdf file is too large to upload. They will only accept 12MB. I don't know how many MB my graphic is, I only know it's too big. How can I reduce the size of MB in my graphic, but keep it the same size (6 X 4)?
If your Illustrator file contains a lot of complex paths, you should try to simplify them by merging them as much as possible.
You can also rasterize some images and background together.
Since you're talking about a postcard, the problem of file size can easily be fixed by exporting in PDF and the file size is way smaller. And there is also an option to keep the file editable:
When you export your PDF, choose the option to downsample the graphics to 300dpi for color images, 600dpi for grayscale and 1240dpi for monochrome/bitmap. You can select the zip compression for all of them but if it's not enough, you can use the Maximum JPG compression. This should already help you decrease the file size of your PDF. If you can avoid using the JPG compression, it's better.
What I personally like to do is to use Acrobat Pro for this and resave my file with the Save as Other/Optimized PDF option in the "file" menu. Usually, a PDF that is 50-100mb can drop down to 1-4mb easily this way, using the save presets I mentioned above. Using Acrobat also help remove unwanted data and "clean up" your PDF if you decide to do so (ex. flatten layers, remove personal data, hyperlinks, etc.) All these things increase a bit the weight of your file. At the same time Adobe Acrobat Pro can be useful to preflight your file and see the CMYK color preview.
Just make sure you don't lower the quality of your file below the required resolution (dpi) your printer can handle!
If your printer really wants to use a .ai file, you can do the steps I mentioned above and then resave that PDF into an .ai file. It should be a smaller file now that all the extras have been removed and the images have been optimized and flattened.