The dc:identifier is commonly an ISBN, though any sort of URN will do. Further information about using in epub can be found on the IDPF's site, but as a quick example, you could use something like:
As long as the identifier is unique, it should be fine. At the above link, the IDPF ...
I am not sure what your two .opf files contain, but the EPUB 2.0.1 specs http://www.idpf.org/doc_library/epub/OCF_2.0.1_draft.doc (section 10.1) say:
The <rootfiles> element MUST contain at least one element
that has a media-type of “application/oebps-package+xml”. Only one
element with a media-type of
“application/oebps-package+xml” SHOULD ...
Personally, I never minify code in ebooks. A big part of the reason for that is that unlike many here, I don't use Sigil or Calibre to edit my epub files—I use either Notepad++ or Geany depending on the OS I'm working in. Because I'm always working with the code, it does nothing but make my life harder if I've minified the whole thing and the client ...
Yes it should be unique, and no, it is not hased to an IP or MAC address.
The example in the EPUB 3.0 specification uses a UUID.
The particular UUID used in the example is:
And the 4 in the position directly after the second - indicates that it is a random UUID. The UUID linked to a MAC address is version 1.
There is ...
A few quick thoughts.
First, if you are using a Kindle Fire tablet, there is no need to do a conversion. just download a PDF Reader app from the app store and read the file that way.
If you are using an eink Kindle, I would suggest sending the PDF to the Kindle Personal Document Service which will do the conversion for you. (It's free and a cool feature ...
This happens because at least two parts of EPUBCheck dislike the ISO8601 timezone format in different ways. In the dc:date context, EPUBCheck is OK with "2020-12-09T14:06:40-07:00" but not with "2020-12-09T14:06:40-0700". The only difference is a colon in the timezone offset of 0700 hours. I learned about this oddity from a PHP manual ...
It's now deprecated in epub3 in favor of the landmark NAV element.
Because support for NAV is pretty much universal in reading systems, I see no reason why to include it now.
To answer your first question. The .opf file holds the metadata that Calibre extracts from the (PDF) file when adding it to Calibre, as well as that metadata that you decide to edit on (author, title, series, etc).
For some formats like ePub this metadata is almost taken verbatim and inserted in one of the files that are zipped into the ePub container. For ...