Why a scan is rarely a PDF right away
Scan a sheet of paper and what you usually get is an image. The office printer drops a JPG on the network drive, the phone app saves a photo, and a multi-page document turns into one file per page. That is fine for your own folder and useless for anything else: send a contract to an authority or a receipt to your accountant, and they expect one document, not eight separate pictures.
Turning those images into a PDF is not a nice extra. It is the step where loose shots become a document.
Several pages in one file
The most common task: five scanned pages should become one PDF, in the right order. That is what Scan to PDF is for. Upload the images, arrange them, save them as a single document.
If the order got mixed up because the feeder pulled the sheets the wrong way round, Organize pages fixes it afterwards. If a page sits upside down, use Rotate PDF.
Resolution: more is not better
People often set the scanner as high as it goes, assuming that cannot hurt. It does hurt, namely the file size. For text documents 300 dpi is the usual value: the type stays sharp and the file stays manageable. 600 dpi only pays off for fine drawings or when the text is meant to be read by software later.
A colour scan at 600 dpi can take several megabytes per page. At ten pages the file is too big for many mailboxes, which often stop at 10 or 20 megabytes. Compress PDF brings it back down without turning the text to mush.
Colour or greyscale
A letter printed in black on white needs no colour. Greyscale often halves the file size with no visible difference. Colour is worth it when stamps, blue ink signatures or coloured markings have to stay recognisable, on a sales contract for instance.
When the text needs to be searchable
A scan is an image, even inside a PDF. You cannot search it and you cannot copy anything out of it. Text recognition is what turns the picture into readable text. PDF text recognition does that after the fact, and it is worth doing whenever the document will be searched or archived later.
For long-term filing
Documents kept for years, tax records for example, belong in PDF/A. It embeds the fonts and leaves out everything that might not render a decade from now. An existing file can be converted with PDF to PDF/A.
In short
Scan first, combine into one document, then check order and orientation. 300 dpi is enough for text, greyscale saves space, and if the file is too big for email, compress it at the end. Every step runs in the browser, so the document never leaves your device.