What PDF layers are
A CAD drawing is organized in layers: property lines on one, existing contours on another, storm sewer, sanitary, text, the title block. When a drawing is exported to PDF with layer information, those layers come along as PDF layers, which the PDF specification calls optional content groups. The linework is all still on the page, but each piece knows which layer it belongs to, so a viewer can show and hide them the way CAD does.
How to view and toggle layers in a PDF
Open the PDF and the layer list appears beside the sheet. Each row is one layer with the name it had in CAD, in the order the PDF lists them, with any group headings the export wrote. Click a layer to switch it off or on. Show all, Hide all, and Reset (back to the PDF's own defaults) are above the list. Move between sheets with the sheet picker; your layer choices stay as you go.
Where it helps
- Reading a busy utility plan: hide the topography and the storm sewer to follow the sanitary line from end to end.
- Finding hidden content: layers the PDF starts with switched off, like minor contours or construction lines, are marked so you can check what was left out.
- Checking a consultant's sheet: confirm that the existing conditions in their PDF are on the layers you expect before converting it or overlaying it.
- Preparing a figure: hide the annotation layers, then save a copy or export an image of just the linework.
Saving a PDF with your layer choices
Save PDF with these layers writes a copy whose layer list opens with your choices: the layers you switched off start hidden, and the rest start visible. Nothing is removed, so anyone can switch a hidden layer back on in a viewer with a layers panel. This only changes how the PDF opens. To remove content for good, the drawing has to be exported again from CAD without those layers.
When there are no layers
If the list says the PDF has no layers, its linework was saved flat. That is common with scans, with sheets printed to PDF from another PDF, and with exports where the layer option was off. In AutoCAD and AutoCAD-based software, the PDF output options include a setting for layer information; an export with it on brings the layers along.
Layers and the other free tools
Layer choices carry into the other tools through a saved copy. Save a PDF with the annotation layers off and open it in the PNG tool for a clean figure of the linework, or in the redline tool to mark up only what matters. The PDF to DXF converter brings every layer into the DXF on its own, with hidden layers arriving switched off.