Ranges are inclusive, and off-by-one is the whole risk
"Pages 4 to 9" means six pages, including both 4 and 9. That is the convention every PDF tool uses and it is still the thing people get wrong, usually when extracting a signed page from a contract and taking the one before it instead.
A related trap: page numbers printed on a document frequently do not match its PDF page numbers, because of cover pages, unnumbered front matter, or a report that restarts numbering at each section. The range you type refers to physical position in the file. When they disagree, count thumbnails rather than trusting the printed number.
What merging does to file size
Merging concatenates content rather than re-encoding it, so the result is roughly the sum of the inputs and the pages are as good as they were. It does not re-compress, which is the right default — silently degrading a scan because two files were combined would be a surprising thing for a merge tool to do.
If the combined file is too large to email, compress it afterwards as a separate deliberate step with Compress PDF, where you can see the before and after sizes and choose the trade.
Rotation is metadata, not a re-render
A PDF page carries a rotation attribute, and rotating a page in a sane tool changes that number rather than redrawing the page at a new angle. That is why rotating is lossless and instant even on a large scan, and why a rotated page is exactly as sharp as it was.
It is also why a page that looks rotated in one reader and not another is usually a reader ignoring the attribute rather than a broken file.
Doing this on a phone
All four tools work on a phone, and the practical limit is memory rather than anything else. A very large document on an older device is worth splitting before you do anything else to it — work on the ten pages you need rather than asking the browser to hold two hundred.
