If I grab a single page of PDF file and drag it into another group, this page will be pasted into this group as an TIFF file that contains three pages of different sizes.
How can I achieve to drag and drop a single page of an PDF file from one group to another pasted into the destination group as an PDF file?
Try: select the page in the PDF’s sidebar, control-click, and choose “Copy” from the contextual menu. Go to the destination group and use Data > New > With Clipboard or ⌘N
(This also works when multiple pages are selected – and they do not need to be contiguous pages in the source PDF.)