Normally pdf files on a computer are associated with the free Adobe Reader, not with Photoshop. You therefore need to install Adobe Reader on your computer which will probably set the association with pdf files for you automatically. If not you will need to set it manually. When you create a pdf using pdf995, or open a pdf file, it should automatically open in Adobe Reader (unless you really want it to open in Photoshop). Having created your pdf it will be saved in the folder that you specified which is where you will find it when you wish to upload to your website. You should not save it from Photoshop.
If your pdf file is very large, that probably means that your chart was very large. Try tweaking your chart to include fewer generations, and experiment until you find the best compromise.