If it had something to do with dynamically loaded parts I might understand, but as @chrillek notes, these are just different letters, nothing special happening there as I can see. But there might be some font trickery happening I don’t understand. Some searching reveals this might be happening more often, especially with combinations with ‘f’: macos - Disable automatic "ligature" handling in PDF/Preview on El Capitan - Ask Different :
When text appears in a TextEdit or Word document, the underlying data maintains the fi pair of characters, but displays them as one glyph: the ligature. If the font doesn’t have the ligature character, then you see both letters separately.
When a PDF is made, the display glyph is used, but the underlying pair of ‘real letters’ is not maintained within the data. PDF was designed as a description of displayed content.
Any PDF that contains an alternative glyph or ligature may not display the correct data when the text is copied and pasted, unless there’s a hidden text layer that contains the ‘correct’ lettering.