The PDF file format is a popular way to make content available on the web in an easy to print format. However, many PDFs used in web sites are designed for high quality print output and are not optimized for the Web; this results in slow downloads and long waits. Also the WMS has a 10Mb limit for library files.
If you have a very large PDF file that you would like made available on your site there are a number of ways you can go about reducing its size. Click here for a detailed article explaining many of these options.