Optimizing a PDF file lets you create a copy with changed properties, more suitable for particular purposes. It allows you to balance file size against display quality.
Select Tools > Optimize PDF for a dialog box with the following panels:
First, specify the compatibility level and choose from compression options.
Optimize PDF for fast web view: Objects are prioritized so texts display quickly and heavy pictures later.
Use Flate to encode non-encoded streams and use it instead of LZW to reduce stream size.
Remove document structure by eliminating tag information.
Remove private data from other applications, embedded thumbnails and invalid bookmarks and links: these can be useless items, they do not affect the appearance of the PDF file.
“Discard all” options:
Comments: Deletes all comments added to the PDF. Comments include notes, markups, drawing objects as well as file and sound attachments.
External cross-references: If the PDF has links to other files that will not be available to other users, these links can be eliminated.
Alternate images: An image in the PDF file can have “alternate” images which are optional and can be deleted. For example a color image can have a gray alternate image which is used for printing instead of the color image. The alternate image is only a plus, the file can be used without it.
These options change the sampling rates, compression methods and quality of color, grayscale and B/ W images in the PDF. To reduce file size, use JPEG or JPEG2000. ZIP can produce bigger file sizes if the original image was compressed with JPEG.
JBIG2 compression is best for black-and-white images. If image quality is less important, use downsampling to reduce file size.
With scanned pages the Use MRC Layers option splits the image into three layers with optimized compression for each, so total file size is reduced. Deskew is useful for images scanned at an angle. Despeckle eliminates spots and shadow lines from scanned pages.
Unembedding a font reduces file size. But the new PDF file may differ from the original; if the viewing computer does not have a required font, it must be substituted by a similar or more basic one.