It's not possible to use the HTML headings (<h1>, <h2>, <h3>...) for auto numbering titles of PDF files generated by Space Tools > PDF Export.

body {
    counter-reset: counterH1;
}
h1:before 
{
    counter-increment: counterH1; 
    content: counter(counterH1) ". ";    
}
h1 
{
    page-break-before: always;
    counter-reset: counterH2; 
}

h2:before {
    counter-increment: counterH2; 
    content: counter(counterH1) "." counter(counterH2) ". ";  
    
}
h2 
{
    page-break-before: always;
    counter-reset: counterH3; 
}

h3:before 
{
    counter-increment: counterH3; 
    content: counter(counterH1) "." counter(counterH2) "." counter(counterH3) ". ";  
}
h3 
{
    page-break-before: always;
}