I tried fcitix and yong (20.0.0s and 18.0.51) on fossapup64_95. They work pretty well most of the time, but I found they cannot be activated on Qpdfview. To test this, try opening a pdf file on qpdfview, click any input area on the tool bar, or press ctrl+f to bring up the text search tool. Fcitx and Yong do not work on these input boxes.
So far I have not tried scim and ibus.
I also did a similar test on Debian 10.6, installed fcitx and qpdfview from Debian's official repo. Fcitx is able to input Chinese in Qpdfview's text search box.