Strictly speaking, it's nearly impossible. Once a file is on someone's computer it can be altered by that person. There are a number of ways, however, to distribute a message which is difficult to alter or which can be falsified if any alterations are made.
One of the easier ways to make such a message would be to use a file format which many people can view but not as many have the right software to edit. The only such widely available document format would be the PDF document format. PDF documents are by no means "read only" but 90% of the time the recipient will only have a PDF reader installed (Adobe Acrobat Reader, Sumatra PDF Reader, etc.)
The other option is to make any changed versions of the document irrefutably obvious that they've been changed. You could accomplish this by several means, but they all boil down to one idea: cryptography.
There exists widely available software, for free, with which you can generate a strong digital key for use in encrypting data. In this case, you make two related keys: one for encrypting and one for decrypting; you keep the encryption key and give away the decrypting. Thereby you could prove that a particular document is either a genuine communique which could only come from you (since you encrypted it with your secret key) or a communique which is demonstrably NOT from you (the decryption key fails to decrypt it.) By the use of various mathematical and cryptographic techniques, any digital data can be "signed" by the author in such a way as it indisputably came from them and has not been altered.
For example, here's my digital signature attesting that I wrote the preceding paragraph:
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.2 (MingW32)
-----END PGP SIGNATURE-----
Anyone who has my PGP Public Key can verify that I wrote that paragraph and that it hasn't been altered.
PDF documents can also support a similar feature by the use of digital certificates.
On the other hand, it would probably be simpler for your wife to send out an e-mail directing recipients to view the message on the organization's website.