some improvments

internal/pdf/core/primitives.go

@@ -3,6 +3,7 @@ package core
 import (
 	"bytes"
 	"fmt"
+	"strings"
 
 	"gitea.tecamino.com/paadi/pdfmerge/internal/pdf/common"
 )
@@ -315,15 +316,16 @@ func (array *PdfObjectArray) String() string {
 
 // DefaultWriteString outputs the object as it is to be written to file.
 func (array *PdfObjectArray) DefaultWriteString() string {
-	outStr := "["
+	var outStr strings.Builder
+	outStr.WriteString("[")
 	for ind, o := range *array {
-		outStr += o.DefaultWriteString()
+		outStr.WriteString(o.DefaultWriteString())
 		if ind < (len(*array) - 1) {
-			outStr += " "
+			outStr.WriteString(" ")
 		}
 	}
-	outStr += "]"
-	return outStr
+	outStr.WriteString("]")
+	return outStr.String()
 }
 
 // Append adds an PdfObject to the array.