PDFRender将PDF文件转为图片出现空白页

我在使用PDFRender将PDF文件转为图片时，发现部分出现空白页，并且提示错误：

java.lang.NullPointerException

    at com.sun.pdfview.font.TTFFont.getOutline(TTFFont.java:170)
    at com.sun.pdfview.font.CIDFontType2.getOutline(CIDFontType2.java:270)
    at com.sun.pdfview.font.OutlineFont.getGlyph(OutlineFont.java:130)
    at com.sun.pdfview.font.PDFFont.getCachedGlyph(PDFFont.java:308)
    at com.sun.pdfview.font.PDFFontEncoding.getGlyphFromCMap(PDFFontEncoding.java:155)
    at com.sun.pdfview.font.PDFFontEncoding.getGlyphs(PDFFontEncoding.java:115)
    at com.sun.pdfview.font.PDFFont.getGlyphs(PDFFont.java:274)
    at com.sun.pdfview.PDFTextFormat.doText(PDFTextFormat.java:269)
    at com.sun.pdfview.PDFParser.iterate(PDFParser.java:752)
    at com.sun.pdfview.BaseWatchable.run(BaseWatchable.java:101)
    at java.lang.Thread.run(Thread.java:745)

打开TTFFont.java:170处是内容：

protected synchronized GeneralPath getOutline(int glyphId, float width) {
        GlyfTable glyf = (GlyfTable)this.font.getTable("glyf");
        Glyf g = glyf.getGlyph(glyphId);
        GeneralPath gp = null;
        if (g instanceof GlyfSimple) {
            gp = this.renderSimpleGlyph((GlyfSimple)g);
        } else if (g instanceof GlyfCompound) {
            gp = this.renderCompoundGlyph(glyf, (GlyfCompound)g);
        } else {
            gp = new GeneralPath();
        }

        HmtxTable hmtx = (HmtxTable)this.font.getTable("hmtx");
        float advance = (float)hmtx.getAdvance(glyphId) / this.unitsPerEm;
        float widthfactor = width / advance;
        AffineTransform at = AffineTransform.getScaleInstance((double)(1.0F / this.unitsPerEm), (double)(1.0F / this.unitsPerEm));
        at.concatenate(AffineTransform.getScaleInstance((double)widthfactor, 1.0D));
        gp.transform(at);
        return gp;
    }

请大神指教应该怎么修改呢？

试一试用Free Spire.PDF for Java来转换PDF到图片呢，参考文章

    import java.awt.image.BufferedImage;
    import java.io.File;
    import java.io.IOException;
    import com.spire.pdf.PdfDocument;
    import javax.imageio.ImageIO;

    public class toImage {

                    public static void main(String[] args) throws IOException {
                            //加载PDF文件
                            PdfDocument doc = new PdfDocument();
                            doc.loadFromFile("Sample.pdf");

                            //保存PDF的每一页到图片
                            BufferedImage image;
                            for (int i = 0; i < doc.getPages().getCount(); i++) {
                                    image = doc.saveAsImage(i);
                                    File file = new File( String.format("ToImage-img-%d.png", i));
                                    ImageIO.write(image, "PNG", file);
                            }

                            doc.close();
        }

    }

看看是不是pdf缺少字体方面的问题。