Thank you for the answer.
I forgot to say that I made a test to reproduce this issue
EXPECT_TRUE(EmbedderTest::OpenDocument("Confidential.pdf"));
FPDF_PAGE page = EmbedderTest::LoadPage(1);
ASSERT_TRUE(page);
// 25% scale
int width = static_cast<int>(FPDF_GetPageWidthF(page) / 4);
int height = static_cast<int>(FPDF_GetPageHeightF(page) / 4);
int alpha = FPDFPage_HasTransparency(page) ? 1 : 0;
ScopedFPDFBitmap bitmap(FPDFBitmap_Create(width, height, alpha));
FPDF_DWORD fill_color = alpha ? 0x00000000 : 0xFFFFFFFF;
FPDFBitmap_FillRect(bitmap.get(), 0, 0, width, height, fill_color);
FPDF_RenderPageBitmap(bitmap.get(), page, 0, 0, width, height, 0, 0);
FPDF_FFLDraw(nullptr, bitmap.get(), page, 0, 0, width, height, 0, 0);
Thanks,
Dmitry