If Firefox wont open 'local' PDF's but offers to download ...
Try this :
1. Open Firefox
2. In the Address Bar type in "about:config" and press enter
3. In the "Filter" field type in "plugin"
4. Look for a line that reads "plugin.disable_full_page_plugin_for_types"
5. If this line exists, right-click on it and select "Reset", or just edit the value to remove "application/pdf", leaving the remaining stuff in place.
If that line doesn't exist, then obviously this fix won't help.
If posting files to this forum - you must archive the files to a ZIP, RAR or 7z file or they will not be uploaded - thank you.