(This is a compilation of some of the comments I provided when the query was first posted.)
Since you're using the plain bibliography style, you could use either the @unpublished entry type or the catch-all @misc entry type for the paper at hand. With either of these two entry types, I suggest you use the note field to provide URL and similar information. Incidentally, based on the description you've provided, I would not use the entry type @techreport for this paper. The @techreport entry type should be used primarily for items issued as part of a working paper series, discussion paper series, or similarly numbered series of papers. That's why the @techreport entry type has fields such as institution (required) and type and number (optional).
Aside: which bibliography style you should -- or must -- use depends importantly on the style guidelines of the journal or outfit you intend to submit your paper to. One of the main advantages of using BibTeX (or, say, biblatex) for generating bibliographies and citation call-outs is that it's straightforward to switch between (pre-defined) bibliography styles.
Regarding your first follow-up question,
Can I always trust the entry generated by google scohlar? Occasionally I found typos there.
As a general rule, you should always double-check the correctness of all entries you've obtained online. In my experience, even bibliographic information derived from the publishers' own websites isn't 100 percent reliable.
You also asked,
I found this description: @Unpublished: A document having an author and title, but not formally published. Required fields: author, title, note. Optional fields: month, year. What is the required field "note"? ... It says that a note is "Any additional information that can help the reader." Then why it is required instead of optional? ... if I write "note={preprint on webpage at math.rochester.edu/people/faculty/cohf}", is it considered enough and suitable?
For entries of type @unpublished, the note field generally contains information about the author's institutional affiliation (if known), a web address, and any other pieces of information which may be important and helpful to the reader and which do not belong in any of the other fields.
If you've loaded the url and/or hyperref packages, you could (should, actually...) encase any URL strings inside note fields in a \url{...} directive. E.g., you might write
note={preprint on webpage at \url{math.rochester.edu/people/faculty/cohf}},
This will help LaTeX find a decent line break in the URL string, should It be necessary to do so.
plainbibliography style, you could use either the type@unpublishedor the catch-all type@misc. For either type, use thenotefield to provide URL and similar information. Which bibliography style you should (or must) use depends importantly on the style guidelines of the journal or outfit you wish to submit your paper to. – Mico Dec 23 '14 at 08:17@unpublished, thenotefield generally contains information about the author's affiliation, a web address, or whatever else is deemed important and helpful to the reader -- and doesn't belong in one of the other fields. – Mico Dec 23 '14 at 08:39urland/orhyperrefpackages, you could also encase the URL string in a\url{...}directive. If nothing else, doing so will help LaTeX find a decent line break in the long string, should It be necessary to do so. – Mico Dec 23 '14 at 08:59bibtex, you should consider using thebiblatexpackage, that' is both powerful and easy to customise. It works with bibtex, but is designed forbiber. Not sure it's relevant to your case, but it also has anonline` entry type. – Bernard Dec 23 '14 at 10:19@techreport. Most journals nowadays mark@techreportas unpublished. – Igor Kotelnikov Dec 26 '14 at 15:07@techreportentry type should be used primarily for items issued as part of a working paper, discussion paper, or similarly numbered series of papers. That's why the@techreportentry type has fields such asinstitution(required) andtypeandnumber(optional). – Mico Dec 28 '14 at 05:26