Press Print and choose Save as PDF as the destination. That is your browser's own PDF engine, so the file is a real PDF with selectable text rather than a picture of one, and it never leaves the machine. There is also a Download as HTML option, which produces a single self-contained file worth keeping for your records.

No. There is no server involved in making the invoice. Everything, including the logo, is held in this browser's local storage on this device. The practical consequence is that clearing your site data clears the draft, so save the file once you are finished with it.

In most countries: the word invoice, a unique sequential number, the date, your business name and address, the customer's name and address, a description of what was supplied, the amount, and any tax charged shown separately with your registration number. VAT and GST registered businesses have extra requirements, and a purchase order reference is often what actually gets an invoice paid, so it is worth including when the customer gave you one.

Usually after, where the discount is a genuine reduction in the price: tax is charged on what the customer actually pays. A settlement or early-payment discount is sometimes handled the other way, with tax on the full amount, and the rules differ by country. It is a choice here rather than an assumption because the two produce different invoices.

Because the currency is set to rupees. The Indian numbering system groups digits in pairs above the first three, so 10,00,000 is ten lakh rather than one million and 1,00,00,000 is one crore. An invoice raised in India that says ten million reads as an error, and one raised in London that says one crore reads as a different error, so the wording follows the currency.

It should not be, and that is deliberate. Each line is rounded to the cent before anything is added, so the printed lines always sum to the printed total. Calculators that add the unrounded values first can show a total a cent away from the lines above it, which is exactly the sort of thing that gets an invoice queried.