Mathcad is where a lot of engineering calculations live: structural design checks, beam and column sizing, pipe and pump sizing, heat loads, electrical cable and fault calculations, lab sheets and homework. The worksheets get sent around constantly, to a checking engineer, a client, a contractor, a lecturer or a classmate, and the person receiving one often has no Mathcad. PTC's software needs a paid licence for full use, the free Express edition is a Windows only install, and Mathcad 15 files from older projects do not open in Prime without a conversion step. Most online converters want the file uploaded to a server, which is not a good idea for a client's design.
This page reads the file itself. A Prime .mcdx file is a ZIP package with the worksheet, the saved results and any pictures inside. McdxOpen rebuilds each maths region from its saved expression tree and shows it as proper typeset maths, with definitions, evaluations, subscripts, fractions, roots, matrices, sums and integrals. Results are the numbers Mathcad saved, shown with their units and the display format the author chose, so 23.33 MPa stays 23.33 MPa. Text regions keep their paragraphs, bold and italic. Pictures, plots and page breaks are placed where the author put them on the page. A Mathcad 15 .xmcd worksheet is plain XML with a saved picture of each maths region, and those pictures are shown as Mathcad drew them, next to the text and results.
The Results tab is the quick way to check a calculation. It lists every variable with a saved value, its number and unit in the order they appear, which is handy when you only need the final design forces, the utilisation ratio or the answers to a homework sheet. Download Results (CSV) puts that table into Excel. Download PDF lays the whole worksheet out on A4 pages with the maths typeset as real text, so you can search it, print it for a calculation pack or attach it to a report. Each plot also has its own Plot data (CSV) and SVG buttons.
Speed was a goal from the start. The reader is a small script of about 46 KB (about 18 KB compressed) that downloads while the page is idle, and the file is read in a background worker, so the page never freezes. A typical worksheet shows its first regions in well under a second, and a 25 MB worksheet with more than 1,600 regions still opens in a couple of seconds on a laptop. Long worksheets are drawn 150 regions at a time with a Show more button, so the page stays quick on a phone.