You can sometimes reduce the file size of a PDF only by using the Save As Other commands. Reducing the size of PDFs improves their performance—particularly when they’re being opened on the web—without altering their appearance.

The Reduce File Size command resamples and recompresses images, removes embedded Base-14 fonts, and subset-embeds fonts that were left embedded. It also compresses document structure and cleans up elements such as invalid bookmarks. If the file size is already as small as possible, this command does not affect.

Note: Reducing the file size of a digitally signed document removes the signature.

1. Open a single PDF, or select one or more PDFs in a PDF Portfolio.
2. Choose File > Save As Other > Reduced Size PDF.
   
   
3. Select the version compatibility that you need.
   
   
4. If you’re confident that all your users use Acrobat or Adobe Acrobat Reader, limiting compatibility to the latest version can further reduce file size.
5. (Optional) To apply the same settings to multiple files, click Apply To Multiple, and add the files. Click OK, then in the Output Options dialogue box, specify your folder and filename preferences.

Note: 
The Apply To Multiple buttons is not available in Pdf Portfolios.
To control changes and quality trade-offs, use PDF Optimizer in Acrobat Pro, which makes more options available.