Many thanks for your kind answer and your efforts to help.
Indeed, I had not realized that the plugin would need Shortcode Core in order to work. Now I have installed it (as well as the two other required plugins), and your plugin is working indeed. I had started using Grav for the first time yesterday, this explains why I had overlooked that.
I have also been able to resize the PDF embed in config.
It is really a nice tool!
Three small things:
1) I have noticed that the PDFs does not show in Firefox (v. 63.03.4). I first assumed that this had to do with my Firefox settings, but embedded PDFs on my WordPress sites show, thus this must have something to do either with Grav or with the plugin. The embedded PDF shows fine on Safari and Chrome.
2) With PDF Embedder on WP, the PDF area is responsive. This does not seem to be the case with the PDF-JS plugin.
3) On your demo page (http://iusvar.alwaysdata.net/grav/en/blog/pdfjs), the PDF area has a nice toolbar on the top. The same is the case with my embedded PDFs on WordPress. However, on my local test site, the toolbar does not show. Is this because it is on my local machine?
Anyway, I agree with you: Grav looks indeed like a remarkable CMS. I have two small sites that I had intended to move to WP (two of my last three sites that are not yet on WP), and I am now wondering if I will not rather move them to Grav.
The fact of having not only a nice CMS, but also developers eager to provide useful plugins and a supportive form, is also something encouraging. Plus the obvious fact that Grav will continue to develop and improve.
Two things that would prevent me - at this point - from considering to move larger sites to Grav (apart from the fact that it would mean a lot of work: redesigning and moving several sites with hundreds of long articles to WP took me months of work!):
1) There seems to be no plugin for managing redirections; for existing sites with a long history, it is necessary to have good tools for the management of redirections (since some URLs change with amove to another CMS), something which is available with WP (e.g. the excellent Redirection plugin).
2) Also missing for the time being is a typography plugin (something like Wp Typography for WP): most of my sites is in French, and a typography plugin is useful for making sure that the specific rules of French typography (small space before some typographical signs, etc,) are enforced.
But I assume that additional plugins will come over time, since Grav is still young, and that such plugins might be available in a not too far future.
In the meantine, I thank you once again for your helpfulness, something which gives me a pleasant impression on my first day of testing Grav.