The last email says “Agreed, thank you,” but contains no file. That does not establish that an earlier attachment has disappeared. A conversation is a group of messages, and the message you are reading may not be the one that carried the document.
Open the message, not just the thread
In Gmail conversation view, replies are grouped and the latest message appears at the bottom. Expand earlier messages to inspect their contents. On the desktop, conversation view can also be turned off in settings if you find individual messages easier to follow. This changes the view; it is not a file recovery operation.
Consider a fictional exchange: Monday's email included outline-v1.pdf, Thursday's included outline-v2.pdf, and Friday's only confirmed agreement. Start with Thursday's message when looking for the revision. Check its sender, date, attachment name, and the actual file contents. A filename containing “final” is not enough to identify the approved version.
If the file is absent there too, use what you know about the sender, subject, and approximate date to locate the original message. A changed subject may put replies into a different Gmail conversation. Repeatedly opening the newest reply will not reveal a file attached elsewhere.
Keep the selected version identifiable
When you download the document, give yourself enough context to distinguish it from older local copies. A folder named for the project and a note of the source message's date can be more useful than another file called “final-final.” Open the downloaded copy and compare an expected change.
Before sharing it, inspect the outgoing draft itself: recipient, actual attachment, and version. Do not assume that visible quoted correspondence means every earlier file is included. If the original sender must clarify which version is approved, ask that specific question and keep the competing versions until it is answered.