awk '
BEGIN {
    rn="MDCLXVI" # Roman numerals desc order
    split("1000 500 100 50 10 5 1",v) # value for each numeral
}
function roman_val(s,val,c,d,p,q) {
    if(s !~ "^[" rn tolower(rn) "]+$") return 0;
    c = split(toupper(s),d,"")
    val = v[index(rn,d[c])]
    while (--c) {
      p = index(rn,d[c])
      q = index(rn,d[c+1])
      val += (p>q)? -v[p] : v[p]
    }
    return val
}
/p_[0-9]+/ {
  x=$0
  while(match(x, "p_[0-9]+")) {
      pg=substr(x,RSTART+2,RLENGTH-2)-16
      n=n substr(x,1,RSTART) "p_" pg
      x=substr(x,RSTART+RLENGTH)
  }
  $0= n x
}

$0 ~ "p_[" rn tolower(rn) "]+" {
  while(match($0, "p_[" rn tolower(rn) "]+")) {
      pg=roman_val(substr($0,RSTART+2,RLENGTH-2))-2
      x=substr($0,1,RSTART) "p_" pg substr($0,RSTART+RLENGTH)
      $0=x
  }
} 1' your_uncompressed.pdf