Pdf preview dpi

app/previewers/high_res_poppler_pdf_previewer.rb

@@ -0,0 +1,38 @@
+class HighResPopplerPdfPreviewer < ActiveStorage::Previewer
+  class << self
+    def accept?(blob)
+      blob.content_type == "application/pdf" && pdftoppm_exists?
+    end
+
+    def pdftoppm_path
+      ActiveStorage.paths[:pdftoppm] || "pdftoppm"
+    end
+
+    def pdftoppm_exists?
+      return @pdftoppm_exists if defined?(@pdftoppm_exists)
+
+      @pdftoppm_exists = system(pdftoppm_path, "-v", out: File::NULL, err: File::NULL)
+    end
+  end
+
+  def preview(**options)
+    download_blob_to_tempfile do |input|
+      draw_first_page_from input do |output|
+        yield io: output, filename: "#{blob.filename.base}.png", content_type: "image/png", **options
+      end
+    end
+  end
+
+  private
+
+  def draw_first_page_from(file, &block)
+    # High-DPI Poppler rendering
+    draw self.class.pdftoppm_path,
+         "-singlefile",
+         "-cropbox",
+         "-r", "300",
+         "-png",
+         file.path,
+         &block
+  end
+end

config/initializers/active_storage_previewers.rb

@@ -0,0 +1,3 @@
+Rails.application.config.after_initialize do
+  Rails.application.config.active_storage.previewers.unshift HighResPopplerPdfPreviewer
+end