Both pdf and odt files of manual pages are automatically produced by the pdf export extension, see Special:Version. There are two issues, which affect the large filesize:

• Firstly, the extension fetches high resolution images from the wiki, to give a print-quality pdf. As far as Bjoern can see, this cannot be configured. Hopefully it'll be possibly to configure this in a future version. It may also be possible to use a templating mechanism to get some choice over image inclusion, but this in terms of usability (for wiki editors) it would be better if the pdf export worked with standard image tags.
• Secondly, the pdf that is generated by the extension cannot be optimised by Adobe Acrobat Professional. When this is tried, an error to do with flattening transparency occurs, and the document cannot be processed.

Unfortunately the combination of these two issues mean that we cannot easily provide smaller pdfs.