A folder of images is awkward to send and easy to lose track of. One PDF arrives in order and opens the same way on every device.
Images of different sizes produce pages of different sizes, which makes a PDF that jumps around as you scroll. Resizing everything to a common width first is the single step that makes the finished document look considered.
Order is not alphabetical unless you make it so. Files named by camera are usually in date order, which is rarely the order you want a portfolio read in, so arrange them deliberately before converting.
Full resolution photos make an enormous PDF. Twenty phone photos at four megabytes each is eighty megabytes of document, which nobody can email. Resize before converting rather than compressing after.
Choose A4 if the portfolio may be printed, or the option that fits the page to each photo if it will only be viewed on a screen. The border defaults to a little over a centimetre, and a larger one gives a gallery feel. A set of landscape photos looks best on A4 pages turned sideways.
Put the photos in the order you want them before you save, because the order cannot be changed afterwards. Use Compress image first if the photos are very large, since each one is placed into the PDF exactly as it is.
There is no limit set by us. Very large sets are limited by your device's memory.
Yes, each image becomes its own page.
No.
For landscape photos on A4, yes. The photo then fills the page instead of sitting small in the middle of a portrait sheet.