No more font or formatting problems after edits
You open a PDF, change one word, and it comes out in a different font — a little wider, a little lighter, plainly not the original. It isn't you. Most PDF files carry only part of their font, and most editors fill the gap with a font of their own.
A PDF stores text as the shapes of a font placed at exact positions. To change the text, an editor needs that same font to draw the new letters. If the font is neither installed nor fully inside the file, the editor has to choose: refuse, or use another font. Most choose another font.
To keep files small, the program that made the PDF usually embeds only the letters the document uses. Type a letter that wasn't there — an “ü” in a name, a “7” in a date — and there is nothing to draw it with.
Install the font on your computer, then edit — if you know which one it is and have it.
Use only letters already in the document — workable for a date, rarely for a name.
Use an editor that finds the same font and takes the missing letters from the same typeface, keeping the line's spacing. Seamtype does this in the browser: when even the typeface has no such letter, it says so instead of switching fonts.
Seamtype edits the text in the document's own font, right in your browser.
Put the pages side by side at high zoom and look along the edited line: letter spacing, the line's right edge, the text after the change. Seamtype does this for you: after each edit it compares the page with the original and undoes the edit if anything outside it changed. Compare then shows before and after on one page.