Sorry, ran out of time yesterday to provide some more details
For quick and dirty images, where screen resolution is sufficient, copy and paste works well enough. You can use windows snipping tool, or the likes of free Foxit reader has a snapshot tool for screen resolution.
If you want a better quality image to import into Designcad, then the Tracker Software pdf x-change editor free version can export images from pdf pages containing mixed content at specified resolution to most any image format. Cute pdf pro also has an image export function, where you can specify your desired resolution.
If you want to import vector content from a vector-based pdf documat, such as you might get by printing to pdf from designcad or autocad, then one of the pdf to cad conversion programs can provide excellent results, which will be scaleable. You might get 2 or 3 digits of precision, where cad itself provides 8 or 16 digits.
So there is medicine out there for whatever needs you may have
