There are many advantages of reducing the size of PDF files:
(a) those who are using homepages server for their web site may have a limited amount of space so reducing file size increases their available space.
(b) Many mail systems do not allow attachments exceeding a particular file size. So for people sending files by e-mails, reducing file size is a necessity.
(c) If files are to be downloaded, whether from the web or from a mail system, it is always convenient if they are of a smaller size, because larger files take longer time to download.
There are several factors that determine PDF file size: image resolution, image type(bitmap or vector), how many fonts are used and how they are embedded, PDF version(Later versions generally produce smaller file sizes), and the level of PDF compression. There are several ways to reduce PDF file size:
(a) re-create the PDF. This way you can remove unwanted objects, tags, and compress the images further. (b) save the file with the save as command. When this command is chosen Acrobat rewrites the entire document as efficiently as possible. But when save is chosen changes are appended to the file, increasing its size. (c) use the least possible number of fonts. You can un-embed fonts if you know they are already installed on the computers of those who will read the document. Even if they are not already installed Acrobat will pick substitute fonts. You can also use subsetted fonts that only include those glyphs actually present in the document. (d) downsampling images help PDF compression by eliminating unnecessary pixel data. JPEG or JPEG 2000 should be used for images in which color changes gradually, ZIP should be used for illustrations with large fields of solid flat color, and JBIG2 should be used for monochrome images.