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Other countries use other materials. Some countries are now using synthetic materials and "plastics"

That is only US money

Also a good copier can be adjusted to print almost perfect money.

Perhaps that is why retail employees use a pen on larger bills.

That pen works on a number of types of paper.

There is a lot of technology that is made into paper money and beyond that make counterfeiting more difficult and easier to identify counterfit money.
Being made out of fabric and linen helps. Perhaps that is why retail employees use a pen on larger bills.

If you scan and printv paper money on a home printer scanner, the result is slightly smaller than the bill.
For you conspiracy theorists, when scanned and printed, the serial number of the printer is encoded in the copy. Some scanners detect that money is scanned and will not scan.

Paper money is actually made from a custom blent of fabrics, 75% cotton and 25% linen. Not paper at all.