Watermark

PDF was designed as a final-form format: the page describes where every glyph sits, not what the paragraph means. Editing tools therefore work by adding a new layer of drawing instructions on top of the existing content stream — a stamp, a page number, a signature image, a rectangle of colour.

Because the addition sits above the original, the underlying text stays searchable and selectable. That is a feature when you are numbering pages, and something to be aware of when you are covering information: a coloured box hides text visually but does not delete it. Flatten or rasterise the page when the content must genuinely be gone.

PDF measures everything in points, where 72 points equal one inch, and its coordinate origin sits at the bottom-left of the page rather than the top-left used by screens. An A4 page is 595 × 842 points; US Letter is 612 × 792.

This is why a margin that looked right on a Letter document shifts slightly on A4, and why placing an element 'twenty from the top' requires subtracting from the page height. Tools that offer margin or position presets handle the arithmetic for you, but knowing the units helps when you need an exact placement.

Any text added to a page needs a font program. If the font is embedded, the document renders identically everywhere; if it is merely referenced, the reader substitutes something similar and spacing can shift. Standard base-14 fonts such as Helvetica and Times are assumed to exist in every reader, which is why added labels commonly use them.

Non-Latin scripts are the usual sticking point: a base font that lacks Cyrillic, Arabic or CJK glyphs will render them as blanks or boxes. Keep added annotations in the Latin alphabet, or convert the finished page to an image, when a document must be reproduced exactly on unknown systems.