Can you recommend (according of your practice) an online (PDF version) remote sensing guide/manual for beginners with virtually no experience in this area.
There are many great books to learn remote sensing from. Some of them have been mentioned already above by others:
Aber, J.S., Marzolff, I., Ries, J.B., 2010. Small-format aerial photography: Principles, techniques and geoscience applications. Elsevier Science, Amsterdam, 266 pp. ISBN: 978-0-444-53260-2
Avery, T.E., Berlin, G.L., 1992. Fundamentals of remote sensing and airphoto interpretation, Fifth ed. Macmillan, New York, vii, 472. ISBN: 0-02-305035-7
Campbell, J.B., Wynne, R.H., 2011. Introduction to remote sensing, Fifth ed. Guilford, New York, xxxi, 667. ISBN: 978-1-60918-176-5
Chuvieco, E., 2016. Fundamentals of satellite remote sensing: An environmental approach, Second ed. CRC Press, Boca Raton, xvii, 457. ISBN: 978-1-4987-2807-2
Elachi, C., van Zyl, J., 2006. Introduction to the physics and techniques of remote sensing, Second ed. Wiley series in remote sensing. Wiley-Interscience, Hoboken, xvii, 552. ISBN: 978-0-471-47569-9
Jensen, J.R., 2007. Remote sensing of the environment: An earth resource perspective, Second ed. Prentice Hall Series in Geographic Information Science. Pearson Prentice Hall, Upper Saddle River, xvi, 592. ISBN: 0-13-188950-8
Jensen, J.R., 2015. Introductory digital image processing: A remote sensing perspective, Fourth ed. Pearson Series in Geographic Information Science. Pearson Education, Glenview, xxxi, 623. ISBN: 978-0-13-405816-0
Joseph, G., 2005. Fundamentals of remote sensing, Second ed. Universities Press, Hyderabad, 473 pp. ISBN: 9788173715358
Schowengerdt, R.A., 2007. Remote sensing: Models and methods for image processing, Third ed. Academic Press, Burlington, 515 pp. ISBN: 978-0-12-369407-2
However, you asked for a free online textbook. Because the books mentioned above do not have a free online version, I can recommend the following online remote sensing resources:
Tempfli e.a., 2009. Principles of remote sensing: an introductory textbook. International Institute for Geo-Information Science and Earth Observation, Enschede, 591 pp. ISBN: 978-90-6164-270-1: https://webapps.itc.utwente.nl/librarywww/papers_2009/general/principlesremotesensing.pdf
Canada Centre for Mapping and Earth Observation, Natural Resources Canada, 2014. Fundamentals of remote sensing, 258 pp.: https://www.nrcan.gc.ca/sites/www.nrcan.gc.ca/files/earthsciences/pdf/resource/tutor/fundam/pdf/fundamentals_e.pdf