I rarely compress PDF files for emailing or any other purpose. For emailing, I very occasionally apply 7-Zip, not so much for its superior compression, but instead for its AES-256 encryption. 7-Zip - Wikipedia One reason that I do not generally recommend applying compression to PDF files is that much of their included information is already compressed. For example, images within PDFs are typically already encoded. PDF - Wikipedia Other information (e.g. vector graphics and text) are also typically somewhat compressed. PDF supports dictionaries for replacing redundant chunks of information with pointers to their dictionary definitions. A more generally efficient PDF compression would involve first unraveling them and applying compressions optimized for each element. For example, compressing text string information (e.g. Huffman coding - Wikipedia), then including a decompression dictionary along with formatting rules for rendering that recovered textual information during PDF playback.
PDF also includes a “storing location” table that may be used by later transcoding instructions. For these and all other reasons, a PDF file in particular is not a good target for “compression-for-compression's sake.” It's a very good practice in my opinion to consider the actual purpose underlying the compression, for example, an attempt to conserve space in order to improve compression. For some tasks, you need to try to preserve as much of a PDF's original content as possible, e.g. to produce a PDF that works on both Mac computers and Windows PCs. For example, if you are converting your PDF file to HTML, then PDFs made from HTML won't work well on PDF-only phones, tablets, web browsers or smartphones. To read this article, you might need to install Adobe Reader on your device to display this article in your browser, or to download the Adobe Reader Application..