You have received (copied) a text book under a certain license, and you can use this material forever (as long as you comply with the terms. The license states "This Public License applies for the term of the Copyright and Similar Rights licensed here. ... " This license does not terminate if the licensor later decides to charge money or ceases to exist.
This is not the right place to recommend tools which allow you to copy or modify content from PDF files. But they exist. So you will be able to create (with some effort) .tex files, and you can create versions (adapted materials) of the book without the trademarks of OpenStax. However, you will still be required to provide attribution to the Rice University.
[Note: Edited below to include issues triggered by the Rice University's specific attribution requirements]
On the webpage, the Rice University states:
Please note that this title is published under a CC BY-NC-SA 4.0
license, which means that you are free to use and adapt, but not for
commercial purposes. Changes you make need to be shared using this
license.
Within the PDF file, the Rice University explains how they would like to see the attribution requirement to be fulfilled: [Note_1]
In any case of sharing the original or adapted material, whether in
whole or in part, the user must provide proper attribution as follows:
- If you noncommercially redistribute this textbook in a digital format (including but not limited to PDF and HTML), then you must
retain on every page the following attribution: “Download for free at
[...] openstax.org/details/books/calculus-volume-1.”
- If you noncommercially redistribute this textbook in a print format, then you must include on every physical page the following
attribution: “Download for free at
[...] openstax.org/details/books/calculus-volume-1.”
- If you noncommercially redistribute part of this textbook, then you must retain in every digital format page view (including but not
limited to PDF and HTML) and on every physical printed page the
following attribution: “Download for free at
[...] openstax.org/details/books/calculus-volume-1.”
- If you use this textbook as a bibliographic reference, please include [...] openstax.org/details/books/calculus-volume-1 in your
citation.
Here starts the problem. While the CC license (Section 3.a.1.A.v) allows the download link, I think this language "Download for free at..." does not qualify as proper attribution, which is usually mentioning the copyright holder of the materials.
Furthermore, it is demanded that the download link is present on every page.
Case 1: If you redistribute this textbook unmodified as a PDF, then you run into the issues that (a) there is a different message present on most of the pages ("This OpenStax book is available for free at [...] cnx.org/content/col11964/1.12") and (b) this message is not even present on every page of the existing PDF [Note_1]. So in order to comply with the attribution format required by the copyright holder, you would need to use a PDF editor to edit each and every page.
Case 2: If you redistribute Adapted Material based on the PDF, where for example you copy chapters 2, 4 and 6 from this textbook while you write chapters 1, 3 and 5 yourself from scratch, then the requirement to have the download message on every page will be confusing and misleading, because that statement is factually wrong, your file cannot be downloaded from the link provided by Rice University.
The requirement for attribution on each and every page is also in contradiction with the CC license Section 3.a.2.
You may satisfy the conditions in Section 3(a)(1) in any reasonable
manner based on the medium, means, and context in which You Share the
Licensed Material. For example, it may be reasonable to satisfy the
conditions by providing a URI or hyperlink to a resource that includes
the required information.
And finally, when you download the textbook, modify it and distribute it further, the recipient is a 'downstream recipient' and you are bound to the CC license without any additional terms. Section 2.a.5.C states:
No downstream restrictions. You may not offer or impose any additional
or different terms or conditions on, or apply any Effective
Technological Measures to, the Licensed Material if doing so restricts
exercise of the Licensed Rights by any recipient of the Licensed
Material.
This means that when distributing the adapted materials downstream, you are not allowed to impose the 'on every page' requirement of Rice University to any downstream recipients.
Conclusion:
You must comply with the terms of the CC BY-NC-SA 4.0 license. Where statements in the PDF are in conflict with the terms of the CC license you can likely ignore these statements because on the website the Rice University clearly commits to the CC license without any caveats.
If you want to be friendly you may contact the support[at]openstax email address, point them to the discrepancies in their offering, and ask for clarification.
Note_1: This language is taken from just one of their files, it may vary in different files.
Please note that this is not legal advice. IANAL, and courts in Texas sometimes have come to strange verdicts in cases related to IPR.