If you need a letter confirming your participation to the conference, then you can request it online from the Registration Tool. However it will only be issued after a registration and payment has been received, and there can be no exceptions to this.
Participants with mainland China passports living in China need to apply a entry permit document approved by the National Immigration Agency to enter Taiwan: Entry Permit Application to Taiwan for Mainland China Passport Holders.
Participants with mainland China passports living at 3rd countries do NOT need to apply the travel Permit for Mainlanders to Enter and Exit Taiwan if they are NOT flying from mainland China, but still have to apply the Entry Permit issued by Ministry of Foreign Affairs of Taiwan. Please read here for your reference.