What deleting a page actually means inside a PDF
A PDF is a graph of numbered objects. The page tree is a hierarchy of /Pages nodes whose leaves are /Page objects; each leaf points at a content stream (the drawing instructions) and a resource dictionary listing the fonts, images, and graphics states those instructions use. There is no in-place "remove" that just deletes a node and calls it done — reliably dropping a page means building a new page tree from scratch, walking the graph for every page you kept, and copying every object each of those pages transitively references into the new document, renumbering references as it goes. That is the same mechanism a page split uses, just applied to the complement of your selection: instead of copying the pages you want, you copy every page except the ones you listed. It is lossless for the same reason splitting is — the copied objects are the original bytes, just relocated into a new page tree.