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.
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.