Slack doesn't provide a way to export or even print important threads that one may want to keep. Here's what I do save threads to PDF.
Open the thread in a web browser and print to PDF.
- In Slack, find the message that's the parent of the thread. In its menu ("⋮", vertical ellipsis), click the "Copy link" item to copy the URL for that message.
- In a web browser, paste the URL copied in the previous step. Authenticate if prompted to do so.
- The browser will try to open the message in the Slack application, if available.
- If prompted to open the the page in Slack (i.e., "Slack.app" on macOS), click "cancel", "disallow", or something similar.
- If the browser opens Slack without prompting, go back to the browser page with the Slack logo and the message, "We’ve redirected you to the desktop app."
- In the browser, click the link for "open this link in your browser". The channel containing the message will be shown in your browser, with the message briefly highlighted.
- If the thread contents isn't shown, click on the "reply in thread" button.
- Select or highlight the entire thread to be saved.
- Double-click to the left and above the user image of the first message.
- Go down to the last message of the thread to be saved and shift-click to the right and below it. The thread to be saved should be highlighted.
- Use the browser or OS command for printing.
- Use the "print selection" option to print just the part of the page that was selected and highlighted in the previous steps.
- Print to a PDF file. If using Google Chrome, it is very convenient to use the "Save to Google Drive" destination.
Feedback and questions are welcomed!