How to convert a scanned PDF to DWG
If you run a scanned drawing through a PDF to DWG converter, you usually get one of two things back: an empty file, or the scan pasted into a DWG as a picture. That is not a bug. A scan holds no lines at all, only pixels, so there is nothing to convert. You have to get the lines back some other way, and there are three of them.
First, check what kind of PDF you have
Open the PDF and zoom in as far as you can on a corner of the drawing. If the lines stay razor sharp and you can select the text with the cursor, it is a vector PDF, exported from a CAD program. AutoCAD's PDFIMPORT command or any decent converter brings those lines across exactly, and you can stop reading here.
If the lines turn into steps of grey pixels and the text can't be selected, it is a scan or a photo saved as PDF. Read on.
Way 1: trace it yourself in AutoCAD
Attach the PDF as an underlay (PDFATTACH) or export the page as an image and attach it (IMAGEATTACH). Find one dimension you trust, ideally a long one, and scale the underlay with SCALE and the Reference option until that line measures right. Then draw over it on your own layers: walls, doors, windows, text, dimensions.
This gives you full control and a clean result, and it is how most drafters still do it. It is also slow. A one-storey house takes a few hours, a set of sheets takes days, and old scans are rarely square, so you end up correcting angles as you go.
Way 2: raster-to-vector software
Vectorizing programs look at the dark pixels and turn them into lines, arcs and polylines automatically. On clean black-on-white line art, such as a crisp scan of a mechanical part, they work well and fast.
On a real-world scan they struggle. Every coffee stain, fold and handwritten note becomes geometry too. A wall drawn as two thick parallel lines comes out as dozens of short segments that don't quite meet. Text is either left as pixels or guessed by character recognition. Expect to spend time cleaning up, and expect no notion of what is a wall and what is a dimension.
Way 3: have it read and redrawn
The third way works like a drafter rather than a scanner: something reads the drawing, understands that this is a wall 8 inches thick, that this is a door, that this number is a dimension, and draws the plan again from scratch in CAD.
That is what Horezmi does with a scanned floor plan. You upload the PDF, and in a few minutes you get a DWG with walls on their own layer, openings, room names with areas and the dimensions taken from the drawing. If something was read wrong, you fix it in words: «the bedroom is 12 feet wide». The catch is that it is a reading, not a copy, so the dimensions need a check before anyone builds from them.
Which way to choose
A rough rule that works:
- Vector PDF: import it (PDFIMPORT or a converter). Nothing else needed.
- Clean line art of a part, a logo, a contour for cutting: raster-to-vector software.
- A floor plan or a building drawing from a scan or a photo: have it redrawn, then check the key dimensions.
- One sheet where every millimetre matters and you have the afternoon: trace it yourself.
Questions
Can AutoCAD convert a scanned PDF to DWG by itself?
PDFIMPORT works on vector PDFs. On a scan it has no lines to import, so you either trace over the scan or use other software.
Why is my converted DWG just an image?
Because the PDF was a scan. Many converters place the picture inside the DWG when they find no vector lines.
How accurate is a redrawn plan?
As accurate as the numbers written on the drawing. Sizes that are written get used as written; sizes that are missing get estimated, and Horezmi says which ones.