Update cheatsheets

pbmreduce

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# pbmreduce
+
+> Proportionally reduce a PBM image.
+> See also: `pamenlarge`, `pamditherbw`.
+> More information: <https://netpbm.sourceforge.net/doc/pbmreduce.html>.
+
+- Reduce the specified image by the specified factor:
+
+`pbmreduce {{N}} {{path/to/image.pbm}} > {{path/to/output.pbm}}`
+
+- Use simple thresholding when reducing:
+
+`pbmreduce -threshold {{N}} {{path/to/image.pbm}} > {{path/to/output.pbm}}`
+
+- Use the specified threshold for all quantizations:
+
+`pbmreduce -value {{0.6}} {{N}} {{path/to/image.pbm}} > {{path/to/output.pbm}}`