Can we say

Has been transfer to another group Hardware development. Though have 10 years PhD(from China), maybe Because the group don't have project for the new comer, maybe my English is not good, maybe I have good reputation of always work hard. I don't get a independent project, has been allocated to 3-4 projects. And all they want me to do is test, document, those work nobody want to do. I have already told my boss i don't want to do test, document. But still get request from boss and co-worker. Maybe this is the best way to get involved in the group and project? Can I say “no”, send email to co-work and CC my boss say “ I could do this part of test, but I will not do all the tests. For according to my current schedule, …

Can I say “no” during the work?
How is the best way to say “no”?
Should I leave here and find another job? Really feel boring and no future. Though the Pay is not bad and the job is stable realitively.