How can I download an extremely long text string from my Iphone

So you are talking about a thread not a string. If this is very important to you, or involves a legal matter, you are going to need to use 3rd party software to transfer them to your computer. If you have a PC use TouchCopy (this also works with Macs), if you have a Mac I prefer Phoneview.

https://www.wideanglesoftware.com/touchcopy/

https://www.ecamm.com/mac/phoneview/

Page1971 Said:

"How can I download an extremely long text string from my Iphone"

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Have a Mac with Monterey?:

Use its Text Recognition: Take a screenshot of the string. Then, transfer the photo to your Mac (eMail it to yourself or use iCloud). Once transferred, open the photo sight the Photos app. What you would then do is select the text and copy it all to an app (i.e. TextEdit).

Page1971 Said:

"How can I download an extremely long text string from my Iphone: The string is literally two years long there are hundreds of texts to take photos of"

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Important:

Back this Up:

Enable Messages for Synchronization: 

1. See this Link: Keep all your messages in iCloud - Apple Support

2. Enable: Messages in iCloud Settings or Preferences across each and every device. If deletion synchronizes as it should be, you are all set.

Is the sentence "If deletion synchronizes as it should be, you are all set" a typo?

Even if they do this while it will add it to their computer if they have a Mac it will not let them do so if they have a PC and if it is a very long thread it will still be difficult to print even from a Mac.