ZWCAD is a popular CAD software known for its compatibility with DWG files and powerful drafting tools. However, users sometimes encounter issues where ZWCAD fails to open certain DWG files and displays the error message "Invalid drawing file." 

Common Causes of the "Invalid Drawing File" Error

1. The DWG file was saved by a higher version of ZWCAD or AutoCAD: DWG files saved by newer versions of ZWCAD or AutoCAD may not be compatible with older ZWCAD versions. For example, trying to open an DWG 2016 file in ZWCAD 2009 can trigger this error.

2. The file is corrupted: Files can become corrupted due to improper saving, interrupted transfers, hardware failures, or software crashes. 

3. The file is encrypted: Some DWG files may be encrypted or password-protected by third-party software, preventing ZWCAD from opening them without decryption.

Solutions to Fix the Issue

When the File was Saved by a Higher Version of CAD Software

Users need to use the corresponding higher version of ZWCAD to open the file or convert the file into a lower version of DWG by the [DWG Converter] tool.

Step 1: Open the DWG file with [Notepad], and check the first few texts for version number.

Step 2: Use the [DWG Converter] tool to convert the file into the proper version. [DWG Converter] is a free application provided by ZWSOFT, allowing users to batch convert DWG/DXF files from a higher version to a lower version, or the other way around. 

When the File is Corrupted

Step 1: Execute RECOVER command in ZWCAD, and choose the file that needs to be opened.

Step 2: Follow the steps to recover the file. If ZWCAD still prompts "Invalid drawing file", it means the file is corrupted beyond repair.

When the File is Encrypted

Contact the drawing provider to decrypt the file.

By following these steps, most issues causing ZWCAD to fail opening DWG files and display the "Invalid drawing file" error can be resolved, allowing you to recover and continue working on your drawings smoothly.