Hi 隸麗從容,
if you wish to open a PDF not with the regular Pleco PDF reader but treat it as an image for Pleco's OCR engine to interpret, you can select Still OCR in the main menu and then Image / PDF File. This allows you to look up any word it is able to recognize in the PDF.
To jump to any page number, you can tap-hold either of the arrow buttons at the top right for about half a second, then release the finger.
There's a test you can do on a desktop computer to see if a PDF is tappable: When you open the PDF and try to select some text with the mouse, if the text is selectable, Pleco's regular PDF reader will very likely be able to look up any word you're tapping on. If you cannot select the text in the PDF, then OCR is needed for at least that part of the PDF.
Hope this helps,
Shun