Depending on the description of the original, if it's complicated I would simply redraw it in 3D from scratch using wireframe line-draw and/or solids modeling software for extrusion, rotation, or any other user-defined operatives on whatever key elements are defined.
Converting to 3D is entirely "show me, don't tell me."
If the original is not too complicated, I would simply convert and import it into a 2D line-draw format then into a 3D wireframe or cad program. I would then rotate two of the three or more orthographic elements to the appropriate x.x-degree angles and move/adjust each node within them along the x, y and z axis until they match the primary dimensions specified in the original drawing. Then I would "fill in the blanks" by drawing the missing lines, arcs, and other elements between the connecting vertices in "snap" mode, avoiding overkill. Most projects don't need a .9999 decimal point exactitude.
Once the 3D drawing is to your satisfaction, if your 3D program allows adding dimension layers, you could add the desired dimensions then rotate it any way you like. But if not, you could rotate it then export it in a 2D format and add dimensions and other notations using a 2D program.
I haven't worked with autocad since version 13, so I'm certainly way behind the eight ball with all the uber-advanced software out there. I have no idea what modern freeware programs might fit the bill, any line draw program that can import/export popular 2D/3D formats would probably work.
There is no single "best" without specifying evaluation criteria and weighing factors. What works well for me will be too much for some, too little for others, or just completely wrong in another application.
The magic bullet that you're looking for, the ability to automatically convert a 2D drawing into a 3D object, isn't there now and probably never will be. Consider the problem of a 2D plan view of a cylinder and a rectangular solid. They look the same, don't they? By integrating that plan view with other elements in the drawing (side/front elevations) or with associated meta-information (e.g., "Figure 21. Right circular cylinder") one can visualize and then create the required 3D shape.
Personally, I've been using Bricscad to build 3D models that are then exported into dimensioned paper-space views for manufacturing. It's pretty easy (and fun, actually) to start with 2D PDFs from vendors of the enclosure, connectors, power supplies, etc. and then build a model for evaluation ("Is this connector in the way of that switch?") and subsequent export into individual manufacturing drawings.
In Bricscad (actually, it looks like you've already found it if that's your forum post over there), the 3D to 2D conversion is pretty easy. Select your solid objects and then use the Quickdraw tool (under Model | X-Solid | Tools) to create the orthogonal and iso views.
I agree. But, for example, the 2D program suggested is pretty darn good for most of us. In general, a very small set of "really good" freeware programs exist for most things we need to do.
There are always two magic bullets needed: a) Conversion of PDF to CAD (e.g., to DWF or DXF) b) Conversion of 2D to 3D
That is the kind of information I need (as testing all the software is beyond our capabilities - so it's always best to follow in your footsteps since you've already tested it a bit for us).
Looks like Bricscad has a free 30-day trial period:
But after that, it seems to be very expensive (compared to freeware) at about $500 to $800 depending on the features. Certainly that's fantastically too high a price for what I need - but the 30-day trial 'might' be worth the learning curve (that is then thrown away) if I can't find freeware to do what I would like it to do.
When I make a 3D object based on a typical PDF datasheet, I usually just use the reader's "snapshot" tool to put a bitmap of the more-or-less (usually "less") dimensioned image from the datasheet into a drawing's model space. Check a few dimensions on the bitmap and scale it as required so the numbers come out correctly. It's rare for a datasheet drawing to have more than a few salient dimensions. The ad hoc dimensioned elements won't be accurate to three decimal places but are good enough for many purposes. (Sometimes it's possible to extract a vector image from a PDF but that often seems to take more time than just grabbing a bitmap.)
The trial version does include the 3D tools for mesh surfaces and ACIS solids and if it's a one-off, that may be all you need. If you're using it professionally, though, it's worth the price. You're also buying the time of a first-rate development and support staff.
That said, I generally recommend to, e.g, people in the office that just need to look at and maybe "fill in the blanks" on a DWG file, that they take a look at 3DS DraftSight, already mentioned up-thread.