Re-lay a PDF onto folding sheets: saddle-stitch booklets, signatures, 2-up and 4-up handouts, and the single-sheet 8-page mini zine with its fold and cut lines drawn on. You get a saved PDF, not a one-off print job. Nothing is uploaded.
Merging puts PDFs end to end. Reordering shuffles the page list. Imposition is a different job: it physically re-lays your pages onto larger sheets, in the order and at the rotation they need to be in so that when the paper is folded, the pages read 1, 2, 3, 4. For an 8-page booklet that means printing 8 and 1 side by side on one face and 2 and 7 on the other, which is not something you can get to by dragging pages around.
This is the fold that zine makers, teachers and tabletop game people keep asking for. One sheet, three folds, one short cut down the middle, and you have an eight page booklet with no staples. The catch has always been working out which of the eight boxes each page goes in and which four of them have to be printed upside down. Pick Mini zine, drop your PDF in, and the arrangement is done for you, with the fold lines and the cut line printed on the sheet so you are not guessing with a ruler.
Creep, sometimes called push out or shingling, is the reason a thick booklet has uneven margins after trimming: the inner sheets stick further out at the fore edge, so they lose more paper when the stack is cut flush. Set a creep value and the inner sheets are nudged back towards the spine to compensate. A spine gutter adds breathing room next to the fold so text does not disappear into the crease. Crop marks land in the outer margin, clear of your artwork, so a guillotine has something to line up with.
Which duplex setting your printer needs is a property of the printer, not of the file. Print a four page test first, and if the backs come out upside down, tick the box that turns the second side 180 degrees and print again. Pages are scaled to fit their cell and every page is scaled by the same amount, so a document with mixed page sizes will have small pages sitting inside larger cells. Encrypted or password protected PDFs cannot be read, and the fold and cut guides are printed onto the sheet, so they sit on the fold and cut lines and end up at the page edges. Only the printed content of each page is carried over, because the pages are re-drawn onto new sheets, so links, form fields, comments and bookmarks are not in the imposed file.
The PDF is opened, re-imposed and saved by your own browser. It is never sent to a server, which matters when the document is a client manuscript, an unreleased zine or anything else you would rather not hand to a web service in exchange for a free conversion.