I hate repeating myself, so the questions below might only have been asked once! There is also a Terminals FAQ.
One of two reasons:
If I don't have the manual, I'd like to obtain it. You can see which manuals I own and offer to fill any gaps for me. I buy manuals, and I'm always willing to accept photocopies or scans.
It can take from one to six months for me to put a manual online, depending on size of manual, number and quality of illustrations, pressure of real work, etc. For this reason, I don't announce in advance which manual will appear next; you'd start checking for its appearance regularly and get disappointed.
The manuals reproduced here are all copyrighted by Digital Equipment Corporation, who were bought by Compaq a few years ago. I have contacted Compaq several times to try to get information about copyright on old terminal and printer documentation. However, I couldn't talk to anyone who could even remember what a terminal was, probably because the terminals business was sold to Boundless Technologies, Inc., and the printers business was sold to Genicom. Boundless told me that they do not own the copyright to old manuals.
The Compaq Assisted Services Catalog still lists terminal manuals for sale, but I was contacted by someone who had tried to obtain the VT100 User Guide from Compaq UK, and he was told they couldn't get it for him.
As no one from Compaq has contacted me in the two years to object to these manuals being online, I have assumed that they are not a big money-spinner and carried on adding manuals.
No. My main aim so far has been to get the text of these manuals online; I really don't care about making facsimiles of the originals for people to print out, particularly as some of these manuals are still obtainable from Compaq. I have had a brief look at producing PDFs, but I will never produce documents in any proprietary word processor format.