Checked against the current Windows build. Where an article names a menu, a button or a message, that is the wording in the application, not a paraphrase.
An embedded font usually holds only the letters already on the page
Fonts inside a PDF are normally subsets. The producer embedded glyph programs for the characters the author actually typed and left out the rest, because there was no reason to carry a whole typeface to draw one page. Type a letter the author never used and the font has nothing to draw it with.
What makes that dangerous is the failure mode. PDFium reports success, the text object measures as though the character were there — a missing glyph still has an advance — and the character is absent from the finished page. Nothing errors, nothing turns red. So Creasepoint does not attempt to re-type new text in an embedded subset. It chooses a face that can draw what you asked for, and then tells you which one it chose.
What your new text is actually set in
If the run was set in one of the standard fourteen — Helvetica, Times, Courier and their bold and italic variants — your replacement is set in the same one and nothing changes but the words. Those faces are never embedded and every reader has them.
Otherwise the run is classified from its font flags and its name, then set again in a Windows system font of the same class: Arial for sans, Times New Roman for serif, Courier New for monospace, with the bold and italic files where the original was bold or italic. Text containing Devanagari through Sinhala is set in Nirmala UI, present on every supported version of Windows and embedded as a subset in the file you save. If the system font file is missing from the Windows Fonts folder, the edit stops and names the file it wanted, rather than producing something silently different.
The note that names the substitution
Every substitution is reported in words, next to the change, before you commit to it. A single-line edit produces a line reading “Run 12 set in Arial because its font PXAAAA+FrutigerLT-Light is embedded, and an embedded font usually holds only the glyphs the author used.” Re-wrapping a paragraph produces the same sentence about the runs it rebuilt.
The notes appear in the tool panel’s results and, because on-page editing applies with a keystroke and no panel open, on the second line of the session banner above the pages. They stop nothing. They tell you where to look, which is the last line of the block, the line beneath it, and anything sitting to its right. While you read them, nothing has reached disk.
A character that never reaches the page
Because a missing glyph fails silently, the finished page is read back and compared with what was asked for, and any character that did not arrive is named. A note might say that Arial has no glyph for the rupee sign, so those characters are not on the page while the rest of the new text is; where nothing at all survived, it says that instead. Up to eight characters are listed before the note ends with “and others”.
This is the case worth zooming in for. A substituted face that looks slightly wrong is cosmetic. A rupee sign, a Devanagari matra or a curly quotation mark that vanished out of a figure or a name changes what the document says, and does it in a way that reads as a typographic choice rather than a loss.
What to do when the result is wrong
Nothing has been written, so the cheapest fix is Undo in the session banner, or Ctrl+Z. Then decide which problem you have. If the face is merely different, and the document is going to a portal that cares about content rather than typography, it is often not worth solving. If the page design is the point — a letterhead, a certificate, a printed form — go back to the file the PDF was made from, because a substituted font is the smallest of the things a PDF editor cannot preserve.
If the text has to be in the original face, that face must exist as a system font on this machine, and the text has to be typed rather than pasted with formatting from elsewhere. Editing PDF text covers the two other ways a replacement visibly breaks a page: a block that grows downwards, and a word too long to break.
The two places worth looking next.
Read the matching guide if you want the reasoning rather than the fix, and check limitations before spending any longer on it — some of what people look for here is absent on purpose.