When I do "open", I see "File Manager", "Morpĥ Browser", etc. I choose "File Manager" and I have to type the password. I tried to do "chmod 755 *" in the terminal (on /home/name) but File Manager still asks for my password.

You can first open the Filemanager, then chose a PDF and chose to view it with Chormium PDF or PDF.js. I very rarely start Chormium PDF or PDF.js directly from the application grid. This way you can also open a pdf directly from Morph Browser downloads, from dekko (e-mails), from Whatsweb, from Teleports, or whatever.

That being said, it is also possible to open first Chromium PDF or PDF.js, then open a PDF with filemanger, but it will display "Transfer in progress" as long as you did not actually select the pdf file in the filemanager window, and validate it with the check icon in the top right-hand corner. Note that it seems that if you already have the file-manager opened in background, it might not necessarily bring the window back in foreground in Noble (i Guess it's a small bug), but you can still go to it and select the file.

When I do "open", I see "File Manager", "Morpĥ Browser", etc. I choose "File Manager" and I have to type the password. I tried to do "chmod 755 *" in the terminal (on /home/name) but File Manager still asks for my password.

You can first open the Filemanager, then chose a PDF and chose to view it with Chormium PDF or PDF.js. I very rarely start Chormium PDF or PDF.js directly from the application grid. This way you can also open a pdf directly from Morph Browser downloads, from dekko (e-mails), from Whatsweb, from Teleports, or whatever.

That being said, it is also possible to open first Chromium PDF or PDF.js, then open a PDF with filemanger, but it will display "Transfer in progress" as long as you did not actually select the pdf file in the filemanager window, and validate it with the check icon in the top right-hand corner. Note that it seems that if you already have the file-manager opened in background, it might not necessarily bring the window back in foreground in Noble (i Guess it's a small bug), but you can still go to it and select the file.

When I do "open", I see "File Manager", "Morpĥ Browser", etc. I choose "File Manager"

Sorry, could you be more clear on what you are doing exactly ? It's sounding as if you are launching PDFJS, then trying to open the file with File Manager.
This would be strange, I'd do the reverse.
In fact I tried it (File manager then selecting the wanted file) and it worked fine (except that for some reason the file manager thinks that opening a Pdf with the Store or the calendar would be a good idea. I tried it for a bit of fun and the calender don't like a pdf file. But PDFJS works. Note that I am using 24.04 though.

I followed the tutorial to install it.