Yes, they can publish a PDF without publishing the modified LaTeX sources. No, this is not DRM.
CC-BY-SA is not an open-source license. It is intended for creative works such as photographs or writings even where there might not be any kind of editable source format.
The license explicitly allows any licensee to change the format of the work:
2(a)(4): Media and formats; technical modifications allowed. The Licensor authorizes You to exercise the Licensed Rights in all media and formats whether now known or hereafter created, and to make technical modifications necessary to do so. The Licensor waives and/or agrees not to assert any right or authority to forbid You from making technical modifications necessary to exercise the Licensed Rights, including technical modifications necessary to circumvent Effective Technological Measures.
The Effective Technological Measures are defined as
1(e): Effective Technological Measures means those measures that, in the absence of proper authority, may not be circumvented under laws fulfilling obligations under Article 11 of the WIPO Copyright Treaty adopted on December 20, 1996, and/or similar international agreements.
In other words, 2(a)(4) allows you to circumvent any accidental DRM on a CC-BY-SA licensed work in order to (re-)use the work as you wish. This freedom to convert formats is important, as it e.g. allows a CC-BY-SA e-book to be converted into a different e-book format, or to be made accessible to blind users, even when the original e-book is “protected” by a DRM scheme. Merely converting a creative work to a format that is non-editable is not DRM – otherwise, printed CC-BY-SA books would be a license violation.
The CC-BY-SA license requires no one to publish their derived works (such as modified LaTeX files), only to license their derived works under CC-BY-SA when they share/publish them. However, they must:
- provide attribution 3(a)(1)(A)(i). If they used your LaTeX files, they must attribute you.
- link to the original work 3(a)(1)(A)(v). When they used your LaTeX files, they must link to them.
- indicate any modifications 3(a)(1)(B). They need not state what they modified, only whether they modified something.
Examples that might clarify some aspects:
I create a CC-BY-SA video that uses CC-bY-SA licensed music in its soundtrack. I am not required to publish the soundtrack in an un-mixed form just so that someone could mix it differently if they wanted to. However, I must link to the original music.
I publish a CC-BY-SA printed book that uses a CC-BY-SA illustration. I am not required to publish my manuscript or layout files so that someone could print the book with a different illustraction. However, I must link to the original image.
I publish a CC-BY-SA “photoshopped” version of a CC-BY-SA photograph. I am not required to publish the project file so that someone could arrange the image layers differently, or remove a special effect they don't like. However, I must link to all source images I used.
If this is not acceptable to you, you should not use CC-BY-SA for your documents.
This answer is based on the terms of the Creative Commons Attribution-ShareAlike 4.0 International license.