some improvments

internal/pdf/ps/stack.go

@@ -1,5 +1,7 @@
 package ps
 
+import "strings"
+
 type PSStack []PSObject
 
 func NewPSStack() *PSStack {
@@ -60,24 +62,26 @@ func (stack *PSStack) PopNumberAsFloat64() (float64, error) {
 	}
 }
 
-func (this *PSStack) String() string {
-	s := "[ "
-	for _, obj := range *this {
-		s += obj.String()
-		s += " "
+func (pS *PSStack) String() string {
+	var s strings.Builder
+	s.WriteString("[ ")
+	for _, obj := range *pS {
+		s.WriteString(obj.String())
+		s.WriteString(" ")
 	}
-	s += "]"
+	s.WriteString("]")
 
-	return s
+	return s.String()
 }
 
-func (this *PSStack) DebugString() string {
-	s := "[ "
-	for _, obj := range *this {
-		s += obj.DebugString()
-		s += " "
+func (pS *PSStack) DebugString() string {
+	var s strings.Builder
+	s.WriteString("[ ")
+	for _, obj := range *pS {
+		s.WriteString(obj.DebugString())
+		s.WriteString(" ")
 	}
-	s += "]"
+	s.WriteString("]")
 
-	return s
+	return s.String()
 }