Welcome Stéphane Guillou, new QA Analyst for LibreOffice (Document Foundation)

A: Haven't specifically used that feature from LibreOffice, but have dealt/provides support for coworkers with PDFs with similar attachments (both editable and other PDFs) and It Just Works (TM), as long as your PDF viewer supports attachments[1]:

On GNU/Linux you likely have at least one of those preinstalled on desktop distros (Evince/Gnome Document Viewer and pals, Okular, Firefox)

On macOS, I'd expect the default system viewer can show attachments

On Windows you need third party software (Firefox, Okular, or one of the expansive proprietary PDF viewers[2]). Notably, Chromium and derivatives (including at least Chrome and Edge) do not support PDF attachments on any platform (!), and I couldn't open them in SumatraPDF[3] (free software, light-weight native Windows PDF viewer).

[1]: On viewers that do not support attachments, the PDF will (well, should) open just fine, but it won't display the fact that it has attachments, won't show you what it has attached, and won't let you recover the attached files

[2]: And yes, I had to fight tooth and nail, on several occasions (which included demonstrations on their own computers via remote desktop), with said Windows-using coworkers (My boss's boss and their secretaries) to tell them that a) You don't need Adobe for this, b) Firefox should be plenty enough, and c) Here, have two alternatives to Adobe which are much less sucky (which they then proceeded to call "Adobe", telling me with extremely smug grins that they had installed Adobe Reader against my vehement wishes, and I wisely refrained to correct them).

[3]: It does display the attachments and lets you click on them, but fails to open them because (in my inexpert opinion) it tries to open them as if each of them were a NTFS ADS (which they are not)