Small improvements to the PDF model

[hoxbro]

I had problems loading PDFs larger than 2 MB. I thought I had fixed this with: #3696

I did, but in #4452 embed was changed from True to False in the Panel class, making the problem reappear. I have updated the Bokeh model to reflect this.

As I already was in the file I did some minor updates to styling.

Also, set the embed src to always have a size of 100% in width and height. So the container is what defines the size.

[hoxbro]

An alternative is we set embed as True for the PDF class.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.89%. Comparing base (9641c9b) to head (4aa9371).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6538      +/-   ##
==========================================
+ Coverage   81.86%   82.89%   +1.03%     
==========================================
  Files         313      313              
  Lines       45931    45950      +19     
==========================================
+ Hits        37600    38089     +489     
+ Misses       8331     7861     -470     

Flag	Coverage Δ
ui-tests	40.08% <100.00%> (+3.04%)	⬆️
unitexamples-tests	71.12% <100.00%> (-0.04%)	⬇️

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[philippjfr]

An alternative is we set embed as True for the PDF class.

No I think this is right.